I made a humble request to the author of MAMEGui, and i actually got a reply!

>Hi :) > >Sure thing. I’m going to upload the Java source code so people can tinker around with it. Right now I’m doing absolutely no work on it >since I’m basically not playing MAME at the moment! :) > >Cheers, >Steve :)

Thanks, Steve! You’re going to make some OS X users verrry happy. :)