Thank you for doing this. Hangtime is my favorite arcade game ever. I would love to see some things in this game get fixed.
I've released mod projects for sports games on SNES, Gamecube, and PS3. I've been working on NBA JAM: On Fire Edition (PS3) for more than 5 years, and have released two projects for that game. I have no experience hacking arcade roms, but I'm sure I can help.
I'd love to fix uniform colors (so many are off, but DET, HOU, TOR, DEN, and VAN are the most inaccurate), add missing players (Jordan Shaq and Barkley are the big ones, but others like LaPhonso Ellis, Dan Majerle, and Dominique Wilkins could maybe be copied from NBA Jam), and change some player ratings (many of the 97 rookies are very overrated).
Here's a doc of mine that lists all the players in both HT and MHT, and what I think are the ideal rosters for both 96 and 97 seasons, in case you're interested:
https://docs.google.com/spreadsheets/d/1PKLBYLZ1FG37mW_lQwm2x9VsPA07GEMtBviGV_r29wI/edit?usp=sharing