Damn, in that case I'm certain that the ones I have gathering dust, all of which came from various packaged mice/keyboards, won't be up to the task. I've got enough old hardware they might still come in handy one day, but I'm pretty sure they're now just relics.

You can buy real PS/2 to USB adapters. E.g. this one from StarTech: https://www.amazon.com/dp/B00028OP2Y

I bought a used slideable rack-mounted LCD last year for my home server rack, and its keyboard with touchpad use PS/2. That's how I found that out.