How would a set of O-rings hold up? If they aren't blown apart right away, they might work, although I'm pretty sure they'll need frequent changing. But then, BP weapons need a frequent cleaning anyhow, so that should not be too much of an issue..
I didn't include the 'shock absorber' idea here because of space issues. Hypothetically, you could still add it, if you can find the right springs. The problem is going to be with material thickness - the already thin push rod would need to be hollow rather than solid, and the pin of the shock absorber would be half the diameter.. that can't be good news for strength.
If it'll break or bend I can't say without testing..
Actually, you can go ahead and test it without doing any mods to the gun. Simply remove the arbor pin and cylinder, and try to push back the hammer with a rod of roughly the same diameter as I used. Keep in mind that in my sketch, the rod is actually fairly well supported: at max push it only sticks out of the arbor pin by an inch. any bending or breaking would have to occur in that last inch.
How long is the arbor pin anyhow? I sort of took a wild guess at around 4 inches or so.. If space allows it, I can probably adjust the design somewhat if breaking or bending is an issue.. I think I could move the end colar back, extend the rear of the push rod in a ticker diameter, move the spring forward.. hmm, hang on, back to the drawing board.. brb