BBBAstards already have a small bubble in the center on most cases. This was sto ensure even mass distribution, and not as a means of moving mass to the outer edge. There is a picture on the home page of the site
if you wait for it to come up in rotation.
Seems like a good idea, but I wonder if the increase in range would be worth the extra manufacturing costs.
There is also a saftey factor here. If you make something more dense it trends toward becoming more brittle, add hollow to that and I believe the structure of the BB would lend itself to shattering.
Most interesting idea tho!