Awesome. I've seen ThatLazyMachinist use brass cylinders. Maybe putting a small flat on the cylinders may also work?
It's said that the ball bearings do indent the work, so maybe I'll try brass or bronze (if I can find bronze spheres).
In any case, the final result should be a nice square piece if work. The ScrewyBall set is sold out on Ebay. The set came in a fancy wooden box, 3 sizes. McMasters sells low carbon bearing balls that they say are machinable.
If you don't want your work indented, couldn't you put a piece of square brass shim on the work? or will this affect "squareness" on the vise?
How are these very hard BB's flat ground ? I don't have a surface grinder and I would like the flat to be, well....flat.