Hey,
With only HMD and motion controllers you don’t get any data that you can really use to solve for the lower body. However, you could get creative in generating task positions for bones outside of the head and hands. You could perhaps do something like drive the hips based on the position of the HMD and experiment with that. Assumptions would have to be made (for example, say the hips and HMD are taken to be always aligned along the Z axis) and would affect your end result but I think it would be very interesting to see the outcome.
If you had a crouching animation already, you could use IKinema on top of that to solve for the head and hands accurately while the player is crouching.