People have shot RHed guns off their Left shoulders for centuries. However, it is safer for the shooter if the gun has the flashhole on the opposite of the gun, for safety. There are double barrel flintlocks rifles and smoothbores made back then, and now, and of course, for both Right and Left handed shooters, you have at least one barrel where the vent is closer to your face, and heading in the wrong direction!
As long as the gun is of good quality materials, and well made, you need only wear safety glasses for protection from the unusual event that something comes back towards your face.
You are more likely to have debris come at you and your forearm from shooting a percussion gun, either sidehammer, mule ear, or underhammer, than from shooting a flintlock. Wear a long sleeve shirt when shooting, when you can. I have the scars from stabs of percussion caps on my forearms to prove the wisdom of this advice.