I'm a lefty.
I use a right hand T/C flintlock on my Chunk/Table shooting rifle. Shooting from a rest I've found no issues at all using the right hand lock. Well, almost no issues..
While working up a competition load from the bench, I found out the hard way how easy it is to forget to keep my right arm out of the path of the ejecta from the touch hole. My right forearm has a nice pattern of those "black spots" well imbedded in it. It only bled for a little while, but from a lot of spots.
When I shoot a right hand flintlock offhand, stock fit, trigger pull weight, sight choice
[old eyes you know] and the quality of the lock are considerably more important factors than
which side the lock is on. Concentrate on sight picture, target image and follow through and the lock-side isn't going to be much of a factor.
All that said...If you build a left hand rifle(cheek piece and cast) with a right hand lock on it, you'll have an oddball for sure, but I've seen them before. Another option would be to build it straight. No cheekpiece, no cast...more "universal" if you will.
As you might can tell...I think about this a lot.
I'm currently looking around for a new flintlock to add to my battery and despite all the "wisdom" I just posted I will admit that I'm wanting to find a totally lefthand build.