I spent more time since my question to figure out the VR subsystem of UE and I have come to the conclusion it is pretty much impossible (without hacking the Engine source code) to prevent the Vive from using the tracking results. UE will always, without a way of turning it off, use the tracking data to setup the projection matrix for the camera.
The LockToHmd option in fact does the reverse from what I expected it to do initially. It lock the position and orientation of the CameraComponent to the position and orientation of the Vive instead of vice versa.
The only way I can think of to prevent the camera from using the Vive tracking results is to add a parent component that performs a reverse tracking transformation. This would effectively stop the camera from updating. Not elegant and probably not entirely perfect (late update might cause some problems), but it might be good enough.