I'm working with what I have...
I have a 65816 computer, and I want a USB mouse.
I have a 6502 computer with USB mouse support.
I have some handy add-in card PCBs for my 65816 that support a VIA.
I have an Arduino Mega with a USB host shield.
The result is USB mouse support for my 65816 computer. I present my latest freak of nature...
Essentially, it's an add-in card with a VIA on the front side and an Arduino on the back side. Riding on the Arduino is a USB host shield. I have the USB connection directed inwards, as I will mount a USB extension cable to the back of the case.
Details of my 6502-based USB mouse: USB Mouse for 6502.
And who needs proper mounting brackets when you have a hot glue gun?! :) Note: I now have the proper configuration for mounts for future add-in card PCBs. My latest PCB, a sound card I just ordered, has the proper mounting holes.