Has anyone who climbed Orizaba in December needed a belay parka, or would a midweight down or synthetic do the trick? Something like an OR Chaos or MH Compressor.

IIRC, for us it was in the 'teens near the top, just before the sun really came up. Medium-weight down parka, heavy mitts, and warm boots made our trip very pleasant.
Can't say what temps you might hit, nor how cold you run.

Have fun.

I had a warm summit day but was happy to have a medium-weight down jacket (MH Subzero). My wife waited at the bottom of the glacier while a friend and I went for the summit and she wouldn't have been able to hang out for a few hours without her down.