You could use joy2key and it allows you to define autofire for buttons...
you can set multiple buttons to the same key, so you could map buttons 1, 2, 3, to 1, 2, 3, and buttons 4, 5, 6, to 1, 2, 3 with autofire between 1 and 30 for each button...
It'd require configuring the keys in the genesis emulator as well, but it'd be worth the hassle.
I hope that helps.
> Looking for something with autofire built in, or maybe someone knows how to hack
> something together with Gens Plus or Kega Fusion?