Results:
Neither of the keyboards I tried worked. Swapping pins didn't help, but swapping the ones I thought were ground and + made the ring of light dance an error message, which I guess verifies that it's (from left to right) Ground, ?, ?, +3v.
So, either it's not USB (which I still think is unlikely), it's USB but anything hooked to the controller needs the chip, or (bet on this one:) it's USB but since I suck at hardware I couldn't get it to work.
Note that I only have wireless controllers. If I had a wired one, the obvious experiment would be to plug it into my computer, then plug various wireless devices into it to see if they get recognised by my Mac. That would allow me to see if it's something to do with that chip or not.