Yeah, UNIF's real win, as near as I can tell, is that it's intrinsically extensible without breaking earlier implementations. I still have some issues with it (using fixed-length strings in something that encodes its own length kind of rankles, but enh).

As for compression, why not just take a bunch of ROM files (of arbitrary format) and run them through a tar/gz library, and let the emulator treat the results as directories? We could call it NEStar and get pun points as well as utility!