I could certainly build an AR-15 upper in .357 magnum without much of a problem, and yes, I would simply sleeve a Desert Eagle magazine into a 20rd GI mag as I do the 44's. I have a bit of a backlog right now but not too bad. The biggest problem in the summer time is that it is so damn hot, and there is no AC in the shop, so I can only work in the early morning hours. This severely cuts back production during this time of the year. I need to either get AC in this place, or move the shop (killer job) to another air conditioned location.
I have only had 2 requests for a .357 in the past three years so there really isn't much demand. It is probably attributible to the steep price tag as well. There were only four (4) uppers ever sold in 44 Magnum and one of those includes the one I gave my dad for Christmas. With this type of demand, its headed for the "discontinued" list very shortly.