I think I've worked out how to do this. You need a couple of scrap pieces of lumber (2x4), two woodworking clamps, a carriage bolt, matching nut & washer.OK, now you've peaked my curiosity!! As a guy that sold hand and power tools for about 30yrs, what drill do you have that you can chuck up a shave brush into? I think to this day the largest diameter I've ever seen listed is a Jacobs Chuck that will accept a 1" diameter shank and I've never seen a shave brush with that small of a handle to fit that!!
Clamp the brush handle between the two pieces of wood. Drill a hole, insert carriage bolt, secure with washer & bolt. Chuck the carriage bolt securely in the drill.
Now, keeping the unbalanced rotating assembly spinning below 500 RPM, let the brush make contact with a concrete floor, brick wall, etc. Your brush should be broken in very quickly.