As far as mapping TIR to an axis in RoF, you can do this. Vander has a post about how to make the Z-axis/lean forward tie into the 'binocular' zoom function. Makes it very natural and improves situational awareness ability. Maybe this isn't what you mean?
No I think you misunderstood what I was getting at. The device I speak of is the EDTracker, I wrote an article here:
http://www.ch-hangar.com/edtracker/ (couldn't link earlier I was on my mobile) It basically uses an off the shelf gyroscope/accelerometer similar to what you might find in a mobile phone these days and through a controller chip it mimics a three axis joystick. So you fasten this device to your head (its the size of a matchbox) - in my case to the headband of my Corsair headphones and when you move your head the gyroscope/accelerometer moves the X/Y and Z axis. You can then map this to head movement in games that support such a feature and away you go - TrackIR like functionality with no need for a camera (not 6DOF though) for £10 (£27 if you buy a pre-made device from the indie mod team who created it).
BoS supports it, as I said in the article BoS has good joystick mapping options, the article contains a list of a growing number of games that support head tracking via an axis, I was surprised RoF didn't support it and hoped maybe it was something they could port over from the enhancements made to the DN engine for BoS.