You can connect an A2000 keyboard directly to the 500's connector with a simple adapter.
It doesn't provide a separate reset signal however, so Ctrl-Amiga-Amiga won't work any more - for that you can either leave the reset line to the original keyboard attached or add a reset switch to the adapter. Alternatively, you can also remodel the reset circuitry from the 2/3/4000, of course.
