I am familiar with slip casting. However, I've never seen it done with something that was completely solid before. I've only seen it done with varying wall thicknesses for things like vases. Could it be done with slip casting? Or could it be done by smashing clay with plaster molds and removing the flash? I need the piece to be solid, because I want to put a hole in the bottom of it to mount it on a sculpture base.
What you want to do is to make paper clay. Basically, you shred toilet paper (cheaper the better) in warm water and use a paint mixer on your drill to make it into pulp. Then add the slip and beat again. Let sump for 24 hours, then place piles of it on slabs to dry. Wedge as usual, and get to work.
This material will change the way you work with clay. It can be solid, because the airpockets that would otherwise explode are used up in the combustion of the fibers. Mounting bone-dry to wet pieces is now possible. Casting works too.