I just discovered this today, and can't believe that I have missed it all these years. It comes from Works of Martin Luther, Vol. III (Philadelphia: A. J. Holman Company and The Castle Press, 1930 / reprinted by Grand Rapids, Michigan: Baker Book House, 1982). I possess this book in my own library.
The individual work is his [Commentary on] the Magnificat (pp. 123-200); translated by Albert T. W. Steinhaeuser, and available online. The translator wrote in the Introduction:
Although Luther regards her in one place as sinless, and invokes her aid and intercession at the beginning and close of his work, these are isolated instances; the whole tenor of the exposition is evangelical, and as far removed from the Mariolatry of Rome as from an ultra-protestant depreciation of the Mother of our Lord. (p. 120)
The Blessed Virgin Mary is described by Luther as sinless (my bolding):
Mary also freely ascribes all to God’s grace, not to her merit. For though she was without sin, yet that grace was too surpassing great for her to deserve it in any way. How should a creature deserve to become the Mother of God!(p. 161)
Three places are noted in the Introduction, where Luther asked for or mentioned Mary’s invocation and/or intercession (my bolding):
May the tender Mother of God herself procure for me the spirit of wisdom, profitably and thoroughly to expound this song of hers, so that your Grace as well as we all may draw therefrom wholesome knowledge and a praiseworthy life, and thus come to chant and sing this Magnificat eternally in heaven. (p. 125)
That is why I said Mary does not desire to be an idol; she does nothing, God does all. We ought to call upon her, that for her sake God may grant and do what we request. Thus also all other saints are to be invoked, so that the work may be every way God’s alone. (p. 164)
Very Catholic! Luther understands biblical paradox: God does all; at the same time (without contradiction) He uses us to do it.
We pray God to give us a right understanding of this Magnificat, an understanding that consists not merely in brilliant words, but I glowing life in body and soul. May Christ grant us this through the intercession and for the sake of His dear Mother Mary. Amen. (p. 198)Therefore, Luther at this time believed in the invocation and intercession of the saints, including Mary. This writing (so says Steinhaeuser) was completed by 10 June 1521 and published in late August or early September 1521.
Later, Luther changed his view on those things, but he still believed them as late as after the famous Diet of Worms ("Here I stand!"), which ran from 28 January to 25 May 1521.