Was William of Ockham the first sedevacantist? Ockham (cf. this article on him by the Catholic logician Paul V. Spade) invented his dead-end nominalist philosophy in order to justify his being against the papacy (cf. Thomist John Deely's Four Ages of Understanding p. 394 ff., which shows how the Great Western "Schism" lead to the adoption of Ockham's nominalism, despite its weakness).

But was Ockham really a sedevacantist (i.e., one not explicitly against the papacy per se but against a particular pope claimant)?

The following quote from Salza &amp; Siscoe's True or False Pope?: Refuting Sedevacantism and Other Modern Errors p. 210 is quite convincing. 

Ockham wrote—"at the end of his letter to the General Chapter in Assisi in the spring of 1334" (cf. Tractatus de Successivis translation p. 12), defending his opposition to Pope John XXII, who opposed the (then-material) dogma that the souls of the deceased destined to heaven behold the Beatific Vision immediately after death, defined by John XXII's successor Benedict XII in Benedictus Deus—that:


  Because of the errors and the heresies mentioned above and countless others, I turned away from the obedience of the false Pope and all who were his friends to the prejudice of the orthodox faith. For men of great learning showed me that because of his errors and heresies the same pseudo-Pope is heretical, deprived of his papacy, and excommunicated by Canon Law itself, without need of further sentence. … In proof thereof several volumes have been published. … For against the errors of this pseudo-Pope I have turned my face like the hardest rock, so that neither lies nor calumnies nor any persecution (which cannot touch my innermost self in any bodily fashion), nor great numbers of men who believe in him or favor him or even defend him, shall be able to prevent me from attacking or reproving his errors, as long as I shall have hand, paper, pen, and ink. …  
  
  If anyone should like to recall me or anyone else who has turned away from the obedience of the false Pope and his friends, let him try to defend his Constitutions and sermons, and show that they agree with Holy Scripture, or that a Pope cannot fall into the wickedness of heresy, or let him show by holy authorities or manifest reasons that one who knows the Pope to be a notorious heretic is obliged to obey him. Let him not, however, adduce the great number of his adherents, nor base his arguments on reproaches, because those who try to arm themselves with great numbers of lies, reproaches, threats, and false calumnies, show that they are void of truth and reason. Therefore let none believe that I mean to turn away from the recognized truth because of the great number of those in favor of the pseudo-Pope, or because of proofs that are common to heretics and to orthodox men, because I prefer Holy Scripture to a man unlearned in holy science, and I have a higher esteem for the doctrine of the Fathers who reign with Christ than for the tradition of men dwelling in this mortal life.


The Church never condemned Ockham's theories, although Ockham was excommunicated for leaving Avignon without permission.

W. Turner writes in the old Catholic Encyclopedia "Ockham" entry:


  Ockham's attitude towards the established order in the Church and towards the recognized system of philosophy in the academic world of his day was one of protest. He has, indeed, been called "the first Protestant". Nevertheless, he recognized in his polemical writings the authority of the Church in spiritual matters, and did not diminish that authority in any respect.


This is sedevacantism because sedevacantism doesn't deny the papacy (as Protestants do).
It's frustrating when an asker of a question then goes on to provide what he or she considers to be the correct answer. Perhaps that is why nobody has bothered to give an answer until now (2 years and 4 months later). After all, what is there left to say when you provide fulsome quotes that appear to confirm your conclusion, namely, "This is sedevacantism because sedevacantism doesn't deny the papacy (as Protestants do)."

Based on the information you impart, I would agree with you. However, I would just point out that in the past, many a Catholic has claimed acceptance of the papacy when suspected of some error in that regard, to save themselves from severe punishment, in this world, not the next. I would also add a few interesting points about William of Ockham, some which partially challenge your account of this man.

He was a medieval English philosopher and Franciscan monk (1285-1349). Basically, one of his philosophical claims was that you shouldn’t make more assumptions than the minimum needed. It can be called the principle or law of parsimony, but this has a long history, predating William  - e.g. Aristotle (384-322 BC) with a claim of the superiority, other things being equal, of the demonstration which derives from fewer postulates of hypotheses. Later, Robert Grosseteste (1168-1253), who greatly advanced the use of experimental methods in science, explained the principle of parsimony as “that is better and more valuable which requires fewer, other circumstances being equal.”

William of Ockham’s near contemporary Thomas Aquinas (1224-74) wrote that, “If a thing can be done adequately by means of one, it is superfluous to do it by means of several; for we observe that nature does not employ two instruments where one suffices.” Ockham advanced the discussion with, “Plurality should not be posited without necessity” and also, “What can be explained by the assumption of fewer things is vainly explained by the assumption of more things.” But the phrase ‘Ockham’s razor’ did not arise till 1852. Although the term is used to advance atheism, Ockham (being a Christian) believed in God as Creator, despite his criticisms of philosophical proofs for the existence of God.

However, when you say that Paul V. Spade claims that William "invented his dead-end nominalist philosophy," that is not the case as he elaborated a new form of existing  Nominalist theory. He rejected the prevailing view the 'universals' really exist. He argued that they are simply artificial products of the human mind, necessary for communicating by means of language. Only individual or 'particular' things have real existence. Since knowledge was based on experience of individual things, natural science took on new significance. 

Apparently denounced as a heretic to Pope John XXII, William was summoned to Avignon in 1324. There, a controversy arose about apostolic poverty, which made him more critical of the papacy. He called for a college of popes to rule the church, and claimed that Christ was the church's only head. He entirely rejected papal authority in secular matters. 

Being "excommunicated for leaving Avignon without permission," as you pointed out, could be a vital clue as to the extent of bad feeling between him and papal authorities of his day. By then he had stressed that God was known by faith alone, not by reason or illumination, and that God's will was absolutely supreme. That would not have endeared him to the papacy, nor helped remove the charge of heresy. Perhaps, after such controlling tactics against him had failed and he was excommunicated, William of Ockham might have switched from being a sedevacantist to then being explicitly against the papacy per se. We do know that he fled, with the charge of heresy still threatening his very life, to the service of Emperor Louis of Bavaria, who supported him in his struggles with the papacy. William died in Munich. 

As for him being "the first sedevacantist", the answer to that question would require deep knowledge of all Christians recorded in history from the time the papacy started until the time William of Ockham became a monk. My answer to that question is simply that he may well have been a sedevacantist (though eventually turned against the papacy per se before he died) but that many Catholics were sedevacantist either privately or simply unknown to Catholic authorities who never wrote anything about them. Given the immense hostility many individual popes aroused even in loyal Catholics, it is impossible for the long era of the papacy, until William's time, to have been devoid of sedevacantists. (Main source, article by Dr. H. D. McDonald, formerly Vice-Principal and Senior Lecturer, History of Doctrine and Philosophy of Religion, London Bible College.)