Using a Raspberry Pi, one or two Arduinos, and possibly a cannibalized PS2 mouse, you can create a device that translates modern USB inputs to PS2 input for a vintage device.
This project is basically a very janky version of USB4VC. USB4VC is a much more polished, well-engineered and maintained product. I built JankPiPS2 before I knew about USB4VC. I am putting it on GitHub in case someone may find it useful.
In the current incarnation there are two Arduinos, although you could probably just use one. One Arduino uses the ps2dev library to emulate a keyboard. The other controls a mouse.
I couldn't get the Arduino to emulate a mouse, so instead, I cannibalized a PS/2 roller mouse. There were two clicky buttons which would click if I brought them up to +5v, and the two quadrature encoders used for rolling. The encoders have four pins: GND, +5V, A and B. A and B would cycle as the ball rolls around. So I brought X-A, X-B, Y-A, Y-B, L and R to pins 4,5,6,7,8 and 9. The Arduino cycles the encoders to simulate movement and clicks the buttons. I actually works great and is much easier than emulating PS/2, BUT it is slightly slower than emulation - it has to "roll" the ball!
If you'd rather have serial mouse input, I have found that USB-Mouse-to-Serial works very well.
So this is about $20 worth of parts not including the Raspberry Pi. If you are very frugal this is probably the cheapest way to connect nested USB inputs to a vintage machine.
Keyboard-driving Arduino on the left with pins 2 and 3 driving PS/2
data. Raspberry Pi, hub pictured. Mouse-driving electronics in a little
box behind the hub.
Mouse-driving Arduino soldered to the zombie mouse's encoders and clickers.
