I've seen a number of modern hammer guns built with safety's, maybe that's what's needed if you don't like the other options. It seems to me that it wouldn't be too hard for a gunsmith to retrofit a trigger interference safety on many of the older hammer guns, of course that might hurt the guns value ("I can't buy this gun" gasp "it has a safety!").
Steve

Approach life like you do a yellow light - RUN IT! (Gail T.)