Wireless USB mouse attached via dongle. Or bluetooth mouse.

If Wifi will go through the Compaq shell, so will mouse signals.

I wouldn't game with it, but keynav (https://www.semicomplete.com/projects/keynav/ / https://github.com/jordansissel/keynav) is pretty great.

I had a Compaq similar to this and it had a trackball that would clip onto the right side. I would love to see something like that hooked up to GPIO.

Very neat project.

There's a USB port on the side of the case.