could I use keyrah to use an a600 keyboard with Fpga arcade?
Sounds like it would work, while also using a USB->PS2 adapter. I'm not sure how useful the USB ports the addon card for FPGA Arcade are to Minimig, maybe it'd work directly, should be able to make it work directly anyway. Or, since it's an FPGA, recode something to put a keyboard adapter directly on the FPGA expansion bus.
But seems like it should be fine. Of course, time, and somebody who has all that in one place, will tell.