can triple 7 be used safely in a 45 caliber side lock?

I can't imagine why not. It tends to give higher velocities than real black, and leaves fouling that is easily cleaned up with a damp patch or two.

I have a can of it sitting on my shelf but haven't opened it at this point. I've chatterd with quite a few m/loaders who do use it and no one dislikes it. Everyone says the loose powder lights off just fine, the pellets come with a warning to only use 209 magnum primers but no one I know uses the pellets anyway.

At any rate, never used it myself but have neither seen nor heard of a reason not to use it, at least in the loose powder formulation.

Was catching up with AR reading when I saw this thread an the note about ONLY using 209 magnum primers with the 777 pellets. That is fine if you like that crud ring and melted plastic from a sabot in your barrel to workout...try .410 shotgun primers...it works well an no crud ring!

I always recommend using real black powder; it is always the best choice for reliability. However, many guys can't get it, and won't clean it. I have also used a lot of 777, but with shotgun primers. Try it with a cap and see it it will work.
As for melting sabots; I must be using better ones because I have never had one melt.