Is Mary omniscient (by participation, not by nature)? I'm mainly interested in an Orthodox or Catholic response. Although any protestant denomination which has a high Mariology is welcome to answer (eg Anglocatholics, high church Lutherans). If your denomination teaches that Mary "was just an ordinary women" then this question isn't for you: please don't bother answering.

Mary is seen as the supreme and perfect example of theosis (to use an Orthodox term). She is said to have maximum justification, maximum sanctification, maximum glorification. She experiences the beatific vision to the maximal extent possible for a created human being. She is as full of God's grace as it is possible to be. Fr Sergius Bulgakov once said that to look at Mary is to look at the Holy Spirit, for she is the most perfect and direct manifestation of the gifts of the Spirit, almost like a second incarnation (but don't worry, he didn't actually go that far and call her a second incarnation).

It is common Catholic teaching that Mary is "Mediatrix of all graces", which means that all the gifts of God to us also pass through her. My understanding of this doctrine is that Mary is up in heaven constantly praying for us, and that her will is so perfectly aligned with the will of Christ that she only prays for things which God desires, and therefore every one of her prayers is directly answered. Therefore, every single grace that God sends to us is also associated with a prayer from Mary, as she is always praying for God's will to be done exactly as it actually gets done. 

Part of theosis is that you become God by participation in the divine nature, while remaining human in essence. Presumably by becoming God by participation you also adopt some of the divine attributes. It seems to me that Mary would share in God's omniscience because she is perfectly divinized and able to pray continuously for exactly those things and everything which God himself desires ("all Graces"). But to do this it would seem that she needs to know everything that God knows, so that she can pray accordingly (otherwise it would be possible for her to pray in a way which is not in accord with God's will due to her ignorance and therefore she would be imperfect and not the mediatrix of all graces)

Does all this mean that Mary participates in God's omniscience?
Whether Mary has no share in the Omniscience of God
Objection 1 it would seem that Our Lady partakes in God's divine nature by the singular fact that prayers addressed to her are proven efficacious time and again;

... pray for us sinners now and at the hour of our death.
... Despise not my petitions, but in thy mercy hear and answer me.
... Pray for us, oh Holy Mother of God; that we may be made worthy of the promises of Christ.

Prayers for the recited in liturgies for the saints saints are usually asked to be heard &quot;In Jesus' Name&quot;.
Objection 2 furthermore, it is necessary to be present to the one to whom we are addressing prayers

Through his Word, God speaks to man. By words, mental or vocal, our prayer takes flesh. Yet it is most important that the heart should be present to him to whom we are speaking in prayer: &quot;Whether or not our prayer is heard depends not on the number of words, but on the fervor of our souls.&quot;
CCC 2700

If your prayers are addressed to Mary, then you must request her presence in the communion of saints. If you recite the &quot;Hail Mary&quot; as a mental prayer, then she must be able to listen to our thoughts.
Objection 3 Mary, is the Channel of Grace (i.e. Mediatrix of all Graces) therefore, she has a share in all divine plans because all grace flows from the source through her.  Therefore Grace flows into our souls through unseen powers that she must be a party to.  Because these powers are invisible and unrestrained, we must refer to them as omniscient.
On the contrary,  Mary's glory is not in being omniscient, or sharing in any of the mighty attributes of God alone.

The Mother of God was above the angels as regards the dignity to which she was chosen by God.
Summa Theologica - 3rd part, Question 30, Article 2, Reply to Objection 2

I answer that even being greater than the angels does not necessarily put her closer to the infinite power of God; just the closest to the infinite power of God.  If we see the Holy Spirit in her, it is because she best reflects the Holy Spirit.

the Virgin Mary is the Church's model of faith and charity. Thus she is a &quot;preeminent and . . . wholly unique member of the Church&quot;; indeed, she is the &quot;exemplary realization&quot; (typus) of the Church.
CCC 967

She is a model for the Church because of this, if she were omniscient, she would cease to be a model and become an idol.
Reply to Objection 1 Prayers to Mary are unique because of her &quot;singular cooperation with the action of the Holy Spirit&quot;.

Mary is the perfect Orans (pray-er), a figure of the Church. When we pray to her, we are adhering with her to the plan of the Father, who sends his Son to save all men. Like the beloved disciple we welcome Jesus' mother into our homes, for she has become the mother of all the living. We can pray with and to her. The prayer of the Church is sustained by the prayer of Mary and united with it in hope.
CCC 2679

This isn't participation in God's omniscience, it is still asking for her intercessory prayers.  But it is on an very intimate level. We trust implicitly that prayers addressed to Our Lady will find favor with God; and it is through the grace of God that this is done.
Reply to Objection 2 clearly if this were true of Mary, it would be true of all the Saints in Heaven who are the subject of intercessory prayer and is the general case of Objection 1 ignoring the fact that prayers to other saints are often done &quot;In Jesus Name&quot; 1..  Ipso Facto, prayers addressed to Mary are a special case - but does it require omniscience to read thoughts?
No, for this is a charism given to the saint by God as evidenced by St. Pio's ability as a confessor to read hearts.

To this task he brought an advantage that few confessors have, the ability to read hearts. Padre Pio's charism enabled him to know when someone was being deceitful in confession or simply had forgotten a serious sin, perhaps through lack of an adequate examination of conscience. In such cases he was able to tel the penitent exactly what they did and when, as well as any relevant circumstances.
EWTN - Padre Pio - Confessor

And no one claimed that Padre Pio could read all souls - just those who came to him in the confessional.  Therefore it doesn't require Omniscience (which is total) to read a soul or hear thoughts.
Reply to Objection 3  again, drawing on the previous answer, Mary as the Mediatrix of all Graces, if proclaimed as the Fifth Marian Dogma might fit in as a 5th charism of Our Lady defined by the Church.  Being the Mother of God and having a Virgin Birth didn't make her Omnipotent; Her Assumption did not make her Omnipresent;  Her Coronation as Queen of Heaven and Earth only confirmed her Benevolence, but did not make her Omnibenevolent. Therefore there is no reason to believe that her role as Mediatrix would make her Omniscient.
However, the confirmation of the would be a great step forward in opening our hearts to God's omnipotence

The fifth Marian dogma would certainly improve the world situation through the prayers of Mary to her divine Son.
...
It will make Catholics realize that just as Mary, through the merits of Jesus Christ, has become co-redemptrix, mediatrix of all graces and advocate for the whole human race, we too share in Mary’s threefold roles for the salvation of humankind.
Zenit Daily Dispatch - Interview With Syro-Malabar Cardinal Vithayathil

1. St. Thomas would have probably posited these in an alternate order