Hi,
Some of you may remember from the new Model F megathread that I have made my own keyboard firmware (USB and key processing implementation from scratch), which also supports the new Model F keyboards. Anyway, one ambition I've had for this firmware was PS/2 output, since I'm also into retrocomputing and would like to use a "Model F" keyboard on vintage computers.
Well, I seem to have finally managed to do it: it just needs two free GPIO pins on the microcontroller, which the xwhatsit/wcass controller has (even conveniently located on a 6-pin header on the side), and either a PS/2 cable/connector wired directly to those, or (what I did) those pins can be wired to the USB connector's D+/D- pins and then one of those old passive USB to PS/2 dongles from the 90's can be used to share the same connector. (The dongles often came with early USB mice, and have the picture of a mouse on them, but they are just passive adapters and the PS/2 mouse and keyboard don't differ electrically at the connector.)
I tested it on a real IBM PS/2 as well as on my own earlier PS/2 to USB converter (yeah, it makes no sense to output PS/2 only to convert it back to USB, but for science (and debugging)), and commercial PS/2 to USB converter. After some initial mistakes and breaking out the logic analyzer (that I seldom bother to use, but really should do more), I think I have a pretty much complete emulation of the IBM Model M PS/2 keyboard that I used as reference, including all three keysets (many commercial keyboards don't even bother to support keyset 3 because Windows doesn't use it (probably because keyboards don't implement it), but I think Linux might benefit).
It's GPLv3 open source of course, but highly experimental at this point. Still, if anyone here would be interested in using it, e.g., for retrocomputing or some other use where it would actually get some testing, I would be delighted to help you set it up. Easiest would be for one of the AVR-based "Brand New Model F Keyboards" where I can tell you exactly where the wires for the PS/2 need to go, but it should in theory work on any AVR-based keyboard that is supported by QMK if you feel comfortable finding the pins on the PCB and soldering to them in case there is no easy-access pin header like on the Model Fs. (I would need to first port the keyboard support from QMK to my firmware, but that should be relatively easy, as I've set up a compatibility layer that takes the key matrix and LED/RGB/EEPROM/etc. access from QMK as is, and then replaces their USB and keycode/layer/macro handling completely with mine).