I'm gonna be ordering a Hori HRAP3 (since I play Melty Blood PC) and I'll also be testing it on 3s MAME. However, the HRAP3 is totally incompatible with PS2s because it has a USB plug and there is no converter in existence that goes USB -> PS2. One option is to get a HRAP2 (it has a PS2 plug) and then get a Pelican PS2-PS3 converter. Lag is minimal, if not nonexistent.
I'd suggest the HRAP2 w/ Pelican option for the compatibility, unless you or your friends have a PS3 with legit games. The only problem would be the price of the HRAP2. It's rare and people know this, so the price is usually $120+. If you're willing to drive to Temple City, you can goto this place called Japan Video Games. I'm not sure if they sell any HRAP2s in their store, but they had a lot of them in their booth in Anime Expo '07 going for $100. Maybe they have extras?
http://www.japanvideogames.com/Default.asp?Redirected=Y