Michael--I use a Golite Adrenaline 1+ season bag. Thanks for picking that up about the Zpacks bivy--I sent a note to them to see about using less expensive fabrics to reduce cost. If anyone happens to have a used one for sale, let me know.
Basically, the upshot is that unless you custom make a bag that doesn't have any zipper at all, it will always be possible to roll over onto your zipper. I don't sleep on my stomach, so I can't say for sure, but I will say that its not an issue to leave the bag where it is and roll over on my side. You do get a face full of bag, but that's life in a mummy bag. I didn't like having a side zip bag because it was very painful to roll over onto, so that was a deciding factor. A center zip is, for most people, the most difficult kind of zipper to end up on top of.