[Cockbrand]

As someone has pointed out in the post's comments, there's the Wombat ADB-USB converter as an alternative. Doesn't look as good, but requires no battery or other power supply. IIRC, it does distinguish between left and right Shift/Command/Option keys. Quickly looking at the product page I realize that it even converts both ways, so you can also use it to attach your USB keyboard to your ADB Mac or NeXT.

[robinsonb5]

TMK includes firmware for an ADB to USB converter which is really easy to build using a 5 volt ProMicro. For the last one I made I just cut an S-Video cable in half and wired it directly to the board. Just needs one pullup resistor.

(Typing this now using an Apple M0116 with salmon Alps keyswitches, using that very converter.)

[Findecanor]

On ADB keyboards with two Command keys, both keys result in the same code being sent — so it is impossible for a converter to distinguish between them.

I have seen that on the AEKII the two keys' switches are even electrically connected on the circuit board.

[Cockbrand]

That I didn't know, thanks!