Oh man this is an awesome idea! I brought this up on another thread and it completely died but thats ok.
That Track IR 5 could be used for right analog. The head movement tracking for looking around. Then some other Track IR 5 deal that watches the legs. The stand still position would be feet shoulder width apart. The walking forward position would be one foot half step forward. sprint would be one foot big step forward.
Crouch and prone would be a little more difficult. Crouch could be done by actually crouching but who wants to crouch for long periods of time(I guess it could cut down on camping

. Prone could be dropping to your knees.
Aiming reticule would be obviously with your hands. The other part as in voice commands would be for the misc. like reload. Grenade you could do my motion. Melee you could do by motion. For squad based tactical games the other voice commands could be to your squad. "Flank to the right" "Rush to cover" stuff like that. The technology I think would be best to look at in the Track IR5. I saw a video of the xyz axis that they used to get that thing to work and it looks right up your alley Obsiv. Ofcourse you'll develop your own deal but I think its very doable.