Annealing the hammer first might be a viable option.
Heat it uber-hot with a MAPP gas torch and let it cool slowly. File slowly until POA is reached then caseharden again by heating it back up to a glowing orange (visable orange in daylight) then quenching in oil or saltwater solution.
You can go one step further to draw out a temper by heating it up again just a few hundred degrees below the casehardening temp and allowing to cool slowly again (air-cool).
This works with many small gun parts and is the preferred method for hardening holes and ejectors on AKM variant receivers.
BTW, I have not tried this on any BP revolver's hammer, so YMMV. I do doubt that sighting the revolver in with an annealed hammer will cause it to peen any, but just to be safe I'd harden it again...
Just trying to provide another option besides ruining your favorite files. The Harbour Freight ones might work, but in my experience you get what you pay for from them. A couple of bucks is worth losing to give it a try first, though.