(Wow, I have a lot of posts to write to catch up on things.)
I've just brought up FreeBSD's TDMA support on the AR9380 chipset. Specifically, the AR9331, since I have a Carambola 2 on me today.
It was pretty simple to bring up - I was missing the beacon configuration HAL call that the TDMA code expected. It's only used by the TDMA code - the STA and AP modes rely on the normal HAL beacon methods that date back to the Atheros HAL.
The only problem - it seems something is up with ANI (noise immunity) and sensitivity on at least the AR9331. It doesn't seem to behave well on slightly loaded channels and thus the beacons don't always go out when they're supposed to.
But, if you've been wanting to play with TDMA on the later Atheros chips, now you can!