Razer Hydra
I managed to try out using my Razer Hydra with opentrack for head tracking. It rocks. You have to move the base station pretty close, but I put mine right next to my throttle and had no issues with jitter. The input was smoother and more accurate than my freetrack setup. It also has no vulnerability to lighting issues and can track at extreme angles. It is kinda a bummer having the extra cords coming off your head, but honestly it's not that bad. It's not that different from having corded headphones. I think I just found my new head tracking setup.
I stand by my assertion that the STEM system would be superior to TrackIR and FreeTrack. Honestly the Hydra is already a better alternative. Considering the Hydra is half the cost of a TrackIR and can also be used on other games as a motion controller, I recommend people start looking into it.