OK, so after mix matching everything, here's the outcome. The 15-22 upper did attach to a new NFA polymer lower. It isn't what I'd call a great fit by any means. This is it with a Chiappa .22 mag.
The Chiappa mag fit with a small amount of play. When using the action, it chambered a round in what appeared to be normal functioning. With the 15-22 mag, there was a lot of play. I'd say 1/4 in or more. It did chamber a round, however it took a little effort. I might have fired a round with the Chiappa mag, but I wouldn't do it with the S&W mag.
Here's the S&W mag in the 5.56 lower. Lots of play. Not likely functional though the mag did stay in place.
At the end of the day, I wouldn't try it if I wanted to ever use my gun again, and liked my face as it is. I tried to put the ATI .22 upper for my DPMS lower on the 15-22 lower, no dice. Simply wouldn't go. And when I put the 15-22 upper on the 5.56 lower, the buffer tube was discontent to stay in its place to say the least.
Final conclusion--Don't do it.