Fortunately the Mac Plus doesn't use ADB, so the mouse is pretty simple. My plan is to start by simulating mouse interrupts with buttons on the board, just to make sure it's behaving as expected. Then I'll make a PS/2 mouse module in the FPGA that receives mouse data and creates fake mouse interrupts. When I make a custom board later, I'll put a DB-9 on it so you can also connect a real M0100 mouse.