With a straight PS2 keyboard (no USB to PS2 adapter) yours was the first case I'd heard of. The first 2 PS2 keyboards I grabbed worked fine for me (One was a Logitech with separate PS2 and USB plugs) and the other was the cheapo eMachines keyboard I eventually used to house my Minimig.
I'd certainly be interested in learning more about this. Perhaps you can list the make and approximate age of the keyboards that didn't work. Also, when you reflash your Minimig with the new core then try one again just to see if there might have been a bug in the early cores which has since been corrected.