Actually, the command string phrase "@0" does not have to refer to physical button 0 on the joystick. In moo_cow.s, the main bit of the high-level object system, there is a table called joybits, in the current example defined thusly:
joybits: .dc.b 24,25"@0" tests the joystick button whose status bit is the number of the first entry in this table, in this case 24, which corresponds to fire-button A on the hacked Nintendo joysticks we are curremtly using. "@1" would choose the next entry in the table, here bit 25. If you need to use more buttons, you just add to the table accordingly. By using this table it should make it easy to reassign buttons according to the whim of Johnny User whilst keeping the underlying logic nice'n'sane.