While searching around, I found some interesting comments from Zwingli's Thought: New Perspectives By Gottfried Wilhelm Locher in regard to Zwingli and the immaculate conception. It says "There is no trace in Zwingli of the doctrine of the 'immaculate conception'..."
The page can be found on Google Books. Footnote #6 is quite interesting, particularly "Thus the holiness of Mary is serviced from that of Christ, and not the other way round." I'm curious as to which conception is in mind when the author states, "Mary was, at least at the moment of conception, wholly pure and free from original sin." With Luther, such a comment usually refers to the conception of Christ. I'm curious if this is the case with Zwingli as well.
Another book states, "Constantly as he insisted upon the certainty of the perpetual virginity of the mother of God he accepted neither her immaculate conception nor her assumption." Another book states, "The doctrine of the Immaculate Conception was disregarded by Zwingli..."
That the early Reformers were devoted to Mary (or held distinctive Roman Catholic beliefs later rejected by Protestants) has been a favorite argument of Roman Catholic apologetics. While the conclusions of the authors mentioned above are intriguing, they are not primary sources. A primary context needs to be exegeted. It does though make me wonder if the same thing is happening to Zwingli's Mariology as that which happened to Luther's Mariology: are Roman Catholic apologists reading history the way they want to, rather than actually tracking down the evidence to see what's there?