DirectVR head tracking means scanning the game‘s memory for the camera rotation/position memory location and then overwriting these with the headset values. That‘s nothing you can do otherwise.
If you can‘t live with that Fallout 3 bug some reason, you can use the vorpX gamepad to mouse/keyboard mapper instead of the native gamepad support also with DirectVR active. Can be turned on and configured to your liking on the ‚Input’ page of the vorpX menu. Personally I would prefer the native gamepad though.