OK got my keyboard adapter really fast! Wasn't expecting it to arrive for another 2 or 3 weeks or more (it's Chinese New Year).
It's this one:
https://www.ebay.com/itm/401253927600
Worked perfectly with my Mitsumi AT keyboard (via AT to PS/2 adapter)!!
But you might have to wait, seller is on vacation because of the New Year. I got really lucky.
Now I need to build the DB-9 ports so I can actually play something, but my tests so far have left me very impressed!
[edit] remember how I said my screen had tearing on 50Hz scrolling? Apparently, that only applies to analog signals. The Minimig core on the Flea is displaying super silky smooth 50Hz screen scrolling on HDMI!!! this is absolutely great, it looks fantastic!
The screen mode is 754x574 50P, weird indeed.