The A1200 board I was using (Rev F?) has the Lyra adaptor for PS2 Keyboard added, and I have built the PS2 Mouse hack, which all works nicely. I added the mouse hack to the 10 pin header on the A1200 Mobo, just behind the mouse port. Used a ribbon cable to the circuit and socket, which is mounted on a plate inserted in a Card Slot position, on the rear of the case.
When I have a joystick plugged into the mouse port, the mouse hack ignores it. But when I have a real amiga mouse attached to the mouse port, the mouse hack stops working.
At least if I wanna 2 player game I don't have to pull the mouse and replace joystick, just leave the joystick plugged in all the time. :-)