Quick update... I have found what I assume is the external keyboard port, a DIN5 next to the cable gland for the pendant controller. A PS2 keyboard and DIN5 adapter will be here on Monday.

I noticed that there is a green LED on the keyboard PCB that flashes when the keys are pressed. But it seems to flash constantly... so perhaps a key is stuck. I'll test removing all ribbon cables for keyboard elements on monday when the PS2 keyboard arrives.