I managed to put a first prototype together: an Arduino nano is talking to a PS/2 mouse. The adapter on the mouse is an old AT to PS/2 keyboard adaptor. There are no electronics in it and I had the 5 pin DIN sockets here. I think I'll stick with that, because the mounting holes for DB9 connectors in the back of a IIe or IIgs are actually prepared for DIN connectors as well.
The A2 can read the XY position register values through a 6522 VIA, because the Arduino is not fast enough to read the address bus and put the information on the data bus. The A2 has to tell the VIA which data it wants to read and that tells the Arduino. That in turn outputs the data that is then latched on the VIA to be read by the A2. Easy, isn't it?
A tiny driver represents the READMOUSE routine of the original card. For now, I can read and display some values in Applesoft BASIC when the mouse is moved.
This pretty much copies the original design. I think the detour through the VIA is annoying and slows down things quite a bit. All registers should be accessible in an instant. My ultimate goal is to get it all in two chips: for example a CPLD and a ROM.