Is there any evidence that Paul was a descendant of Jonathan? Paul being a descendant of Jonathan would have some appeal from a devotional perspective since Jesus' more direct saving of Paul could be viewed as fulfilling the covenant of friendship between David and Jonathan and their descendants (1 Samuel 20:42).

From Philippians 3:5 we know that he was from the tribe of Benjamin (like Jonathan) and Paul's other name, Saul, might be more common among descendants of King Saul than among Benjaminites generally. On the other hand, with the purging of the house of Saul (2 Samuel 9:3 indicates that Mephibosheth might be the bottleneck as a sole survivor) there might have been few if any descendants of Jonathan in the first century A.D.

Is there any other evidence supporting or falsifying this possibility or is this merely a wild speculation where even tradition is silent?

Optional bonus question: Has this speculation been written about earlier in Church history? (Allegory and other somewhat fanciful conceits seem to have been more popular earlier in Church history, so I would not be surprised if someone had considered this possibility given its devotional attractiveness.)
I'm not sure if this question has arisen in Christendom. As, most of Christendom is fascinated by Moses, King David, Jesus, Peter, and Paul. I'd say it's 'unlikely' but...there are billions who have gone before us. :) I'm sure it's come up before but I'm not sure that the question has been documented.

As to the question itself. My current understanding is no jew desired to keep a detailed tally of their heritage, with exception to the descendants of David (Luke 2:4) since they were ancient jews, it was their conviction that the Messiah would come through the line of David (2 Samuel 7:12-16). The only identification they, all Israelites, used are two fold. Whom was their parent and what tribe did they come from? As you can see this thread throughout scripture.


The prophetess Anna, daughter of Phanuel, of the tribe of Asher (Luke
2:36)  
king Saul, son of Kish, a man of the tribe of Benjamin (Acts
13:21)  
Bezalel, son of Uri, grandson of Hur, of the tribe of Judah
(Exodus 31:2)  
Oholiab, son of Ahisamach, of the tribe of Dan (Exodus
31:6)


If you want a lot of these examples, go here.

In conclusion, someone throughout the ages could've asked this same question but I haven't seen it yet. Also, the jews simply wouldn't ask this question because they were focused on the Davidic line to find the Messiah.