Cartridges of the world lists "Trapdoor-safe" loads with a 300g bullet as having in the area of a 1400 fps velocity. Hornady's loading manual also lists "soft" loads in this velocity. You could check the advertised velocity of a commercial round as a starting point for figuring out whether it's safe in your rifle.
If you're not a reloader, what you might consider is calling the manufacturer about the ammunition in question and ask what pressures that particular load is rated at. "Cowboy loads" in .45-70 would probably be a good place to start. It's always a good sign if the ammunition box itself declares that it's rated for Trapdoors.
I didn't mention the "soft" smokeless loads in my first post, as has been pointed out, but I wanted to emphasize the (comparative) weakness of the Trapdoor next to modern Marlin and Ruger rifles. My biggest fear with the old .45-70s is that someone will stick a random load in the chamber, and touch off a grenade. With proper research, though, you can certainly find "Trapdoor-safe" smokeless loads.
Were that gun mine, I'd certainly shoot it, after finding (or loading) proper ammunition. Keep it within its limits, and it'll last another 140 years.
