I think you have the wrong idea about the hammering with a rivet gun based hammer. You hold the metal on the lower die, the upper just rests against the subject metal. Inside the rivet gun, there is a piston that moves up and down hitting the upper die. So, yes, if you get your fingers in there, it will pinch. However, you'd have to work at lifting the upper die (it will only move a little) and getting your finger in there.
Now with a typical power hammer, trip hammer or helve hammer, it lifts the upper head off the material and slams it back down - so finger pinch is a real concern.
I'd look at a small rivet gun, 1x to 3x size, perhaps Kent's dies, perhaps home made to replicate the hammers you are using. Of course, regulator for air control.
Kent has done brass, copper and art work. If you look at his website
www.tinmantech.com you'll find where you can send him a question and he'll get back to you.
Might not be the answer, but it's a lead.