Yes, I do mold some with a void.
If you needed 25 ml of resign to "fill" the mold, you could mix up 15 ml and when you pour into the mold you then rotate and turn the mold for 6 or 7 minutes until it sets up. I normally wait about 3 or 4 minutes before I pour then I don't have to rotate or turn the mold as long.
Because you "rotomolded" the item, you have a void left in the center of the item. In the example above, you would have a 10 ml void.
I do this on one of my lures but I use a fraction of the Alumilite White it would take, then I fill the core with 610 Alumilite Superfoam. It leaves me with a great surface, a great floater, but a couple of sprues to fill and seal (pour hole and expansion hole).
Now, back to the Alumifoam. Mix it up and quickly pour it into the mold, very quickly. Then rotate and turn the mold like I explained above. When it fires off you get a perfect skin, no holes to fill, no bubble in high heat.
Have fun and let us know how it works for you.