there was that version of freetrack that supports trackir cameras. natural point through a hissy fit over it and got them to take that feature out. but everyone that uses freetrack made a point to archive that build. id be using a trackir through freetrack. the freetrack api is pretty widely supported (being open source). thats how i was able to get at the camera vars through lua.
not sure what other head tracking softwares are out there now. i know facetracknoir is compatible. ive been curios about mems gyro based tracking for some time, probibly what vr is using. i did get a useful if not fast (used an 8-bit micro) inertial reference from a $10 imu breakout bored, stick that on an esp32 (would solve the speed problem) and you got yourself a wireless tracker. another option, theres a lot of camera+fpga breakout boards in the wild, thats pretty much what trackir is, some verilog and you got you got spot detection to keep your usb throughput low. all these things could work with the freetrack api.