The billets are annealed. My understanding of your description of your goals is that you want to create a concave surface on the 2" x 2" surface so that a slumped vessel will have good contact with it. To make that surface, some of the glass will need to move somewhere else to make that space, so I think you would need to go to a higher process temperature than the temperature you would use to slump, and dams would be necessary. Does that make sense?
If I were doing this, I would fire the base upside down. I would create a mold to fit the bottom of the bowl and put it as a convex shape in the bottom of a box mold made of dams or vermiculite. Fired to a full fuse so that the top surface stayed flat.
Or, I would make sure my slumped vessel had a flat base--then you could use the billet chunk as is, without all this extra trouble!