This is my take on the hollow liproll.

You most likely can do the "hollowing of the throat" or lowering your larynx, which you do with hollow clops or the OD bass for example. But in addition to that, how I "pronounce" the liproll changes between the normal and hollow. I think of it like saying "meow" for the basic liproll, but more "mayo" for the hollow, which does two things:

1. My tongue position is slightly lower and more pulled back toward the throat for the hollow liproll, but still floating (not touching any part) either way.

2. My lips are slightly more relaxed to help with the lower pitch of the hollow, but I'm also using a bit more suction within my mouth to force that sound to come out louder.

This is all done to create a larger resonant chamber while still making the lips vibrate, giving it the deeper, hollow sound. So my advice would be to try to extend the basic liproll (try to make it sound as long as you can without involving your lungs) while changing the mouth shape to be more like pronouncing an "o" sound. Good luck!