Ultra Burst does not "barely resemble" mega evolution, it is identical in almost every way. The difference is that you use a Z crystal instead of a stone (which doesn't matter since the things that can use the corresponding Z move will likely be banned from holding stones), and that the stat boost is not 100 (just like gen 6 mega Alakazam). It is more similar to Mega Evolution than Primal Reversion is.
I'm not adding a new mechanic for one forme change method/one Pokémon. This metagame is already complex enough for new players, it already has the most mechanics of any OM, it's already the most complex permanent ladder format by far. By adding this shininess concept you have suggested, not only does it not abide by the standards in which we hold metagames, since this is a very specific rule but it also doesn't make any sense whatsoever. It would make more sense to include Griseous Orb than Ultranecrozium Z.
Anyway this stops here. We've had enough back and forth discussion on this issue. If you still believe that it should be implemented and Mix and Mega should become a pet mod, then by all means contact me on PS or Discord and we can discuss it there.