Hi Mike,
this should be possible either with lasgrid64 or lasvoxel, probably with more than one step (count of all grid items, count of grid items in z range), then subtract both values.
We are not complete sure what exactly you want to have as result.
If you are not able to solve it: Please provide a sample and a exact description what you want to have as result.
Cheers,
Jochen @rapidlasso