You could probably do it pretty easily with a disassembler or a debugger.  The buttons are read from a single memory mapped register at 0x4000130, so searching (in a disassembly) for reads from that address or setting a read breakpoint (in a debugger) on it should allow you to locate the routine or routines that read the keypad.

Once the code is located, you may be able to simply hack some bitmasks to make the game treat the A and B buttons as though you've hit L and R instead.

Alternatively, you could trace the code from that point and locate the routine that acts on the button presses and change the conditionals there. That would be much cleaner and the proper way to do it, but it'd also be more work. Although, this does have the advantage of not messing with non-combat button mapping. (Provided they use the same input routine. If they don't, it won't be an issue.). It could possibly also allow you to locate the combo tables and stuff, which could make for some more interesting hacks.