How to track the actual viewpoint

[Jerry]

When tracking the position of the observer's viewpoint using
the PPT tracking system, ideally the tracked LED should be
located between the observer's eyes since that is where the
egocentric viewpoint is located. However, this is impossible to
achieve and so the LED is usually fixed to the HMD at some
position offset from the viewpoint. I realize a preTranslation
of the link between the PPT and the viewpoint can be used to
initially place the tracked location between the eyes, but
if the observer rotates the head that offset is no longer valid.

Is there some way to combine the PPT and an orientation sensor so that
the viewpoint can be placed between the eyes despite translations and
rotations of the observer?