Arabella was VIDC1. The original ARM chipset were names:
ARM2: I forget (VL86C010)
VIDC1: Arabella (VL86C310)
MEMC1: Anna (VL86C110)
IOC: Albion (VL86C410)
I very much doubt it has an ARM1, since they were missing the multiply instruction so wouldn't run RISC OS 2.
Assuming your A500 has a real Albion (IOC), it should speak the same KATN protocol as normal Archimedes keyboards. So you should be able to attach one of those. Didn't someone (Atomwide?) make a PS/2 to Arc keyboard converter in the early nineties?