To follow up...

By typing "rm /etc/config/wireless" at just the right time, I was able to break the reboot cycle.

So my next question is: apart from connecting GND to a few (but not all) pins, a 3.3V power source, 3.3V level RX and TX, and a 2.4 GHz antenna - is anything else needed to make a minimal Carambola 2 setup work, including wireless?

(I'm still interested in a better way to recover from the boot failure cycle, the above was really just luck)

Yes correct.You need GND, 3.3V and antenna.

I can;t get wireless to work, even just using default network & wireless files and just enabling wireless with the original ap settings causes the C2 to reboot (looks like it happens around the time wireless gets activated).

Is there a way to reset the C2 as configured to the original file sets to rule out any mistake?

The other option I guess would be to upload a new image through serial, but that looks like a risky job. I don't have wired ethernet, this is a bare C2 board.