To manually set the A button of the first player as pressed for one frame (PAL, untested).
Code:
.set InputManager_s_instance, 0x809bd70c
lis r3, InputManager_s_instance@ha
lwz r3, InputManager_s_instance@l (r3)
lwz r3, 0x8 (r3)
lhz r4, 0x20 (r3)
ori r4, r4, 0x1
sth r4, 0x20 (r3)