I've recently ported coreboot on MBA 5,2 (13'' mid 2012 model) and MBP 10,1 (15'' mid 2012 retina model). The integrated keyboard on these models is connected as a USB device, not PS/2. I have tested GRUB, SeaBIOS and Tianocore payloads, and the keyboard works in GRUB and Tianocore but not in SeaBIOS.
I was advised to send a log to this mailing list. It was created with SEABIOS_DEBUG_LEVEL=9. I'm not yet familiar with SeaBIOS code, but with your help perhaps I can debug and fix this.