What Willie said.
Something is gonna give; either the frame or the hammer, and the softer metal will peen over.
I would suggest making some snap caps. You could do this by drilling out small pieces of wooden dowel to fit snugly over the nipples, and using a leather punch, snip out small plugs of rubber or silicone sheet the same diameter and glue them to the back side of the snap caps. This should cushion the blow. With any luck, you won't shatter the dowels or drive pieces into the nipples.
(I've got this all figured out, but have yet to try it.) :idunno: