Hopefully to Apple Engineers/Support reading this…has there been any updates this year to allow alternate app icons to be Icon Composer files?

In case you're still trying to figure this one out, or in case anyone else has found their way here with the same problem. All you need to do is select your icon and then in the Inspector select "Single Size". WWDC22's "What's New in Xcode" had a short section covering this new feature. Here is a direct-time link for you: https://developer.apple.com/wwdc22/110427?t=1219