I'm involved in a free 6-degrees-of-freedom webcam-based headtracking program called Freetrack, http://www.free-track.org, and due to popular request I'm implementing FS2004 support. I've managed to get FSUIPC to control yaw, pitch and roll in FS2004 using the AXIS_PAN_HEADING, AXIS_PAN_PITCH and AXIS_PAN_TILT controls respectively (very easily). I also want to get an additional 3 degrees of freedom (translation), I tried using the eyepoint controls, specifically EYEPOINT_UP, EYEPOINT_DOWN, EYEPOINT_RIGHT, EYEPOINT_LEFT, EYEPOINT_FORWARD, EYEPOINT_BACK but these are simple on/off keys and the results are, as you would expect, poor.
According to FSUIPC docs the viewpoint offset cannot be written to, which I tested and confirmed. But it is possible to write to the tower (and to a less extent the spot view), which is a bit odd.
I've searched for other FS2004 view control programs, like ActiveCamera, but none appear to be able to write the viewpoint position in virtual cockpit mode (ActiveCamera just uses the eyepoint keys).
However the TrackIR FS2004 module, under the company name GoFlight Inc. is able to do it, so it IS possible...