Clark Custom Guns used to offer powder coating on firearms and there was even an outfit here in Mississippi that tried it for a while. I saw them at a gunshow several years ago, and they had a blaze orange shotgun and some other brightly colored items to show their capabilities. I couldn't find anything about them in a search, so I am guessing they are no longer around.
I personally think the only issue would be the tolerance stacking you mentioned on fitted parts. For exterior parts it should be good to go. 350 degrees shouldn't affect any metal parts except maybe springs, and you wouldn't be powder coating them anyway. I have used Brownells' Teflon/Moly Oven Cure Gun Finish (
http://www.brownells.com/.aspx/pid=1145/Product/TEFLON_MOLY_OVEN_CURE__GUN_FINISH) with pretty good results and it requires baking at 350 degrees F for 30 minutes.
I am pretty sure that the IonBond on my Limited gun required 400 degrees F, although it may have been 300. I've read that finishes like Melonite and Glock's Tenifer require 1050 degrees F, but that is why they can't be applied to firing pins, sears, etc.