I was thinking about this question. Not until I read the OP's question though. Just never came up. Sort of a "non thought." Then I thought about it. Don't have a clue why. I have known several folks over the years that shot SRCs in CAS. Can't remember anyone having problems with the "ring."
Now, understand, some of the folks with SRCs had carefully staged their rifles so the ring stood the rifle up off the prop table to make the wrist of the rifle easier for a righty to grasp. Securely grasping one's rifle seems to be a really great idea. Of course, sometimes the rifle was "moved" by Poltergeist whom either caused the rifle to slide forward or backward, collapsing down off the ring. What a shame. But I digress. Moving right along ........
The Saddle Ring and "Staple" are forward of the wrist. No, not the shooters wrist (well, that too) but the rifle's wrist. Because the staple and ring are completely forward of the wrist where one might conceivably grasp the rifle for the act of aiming and firing said rifle, I personally don't see where the SRC could be any form of a die fucultie for a leftie. Perhaps.
Of course, because of the location of the staple and ring being on the near side (as opposed to the far side) for a leftie, the possibility of staging the rifle "up" on the ring aren't even an opportune option. Nope, stuck with the rifle staged .... flat.
So ..... in summation, the Saddle Ring Carbine is neither at advantage nor disadvantage to a leftie. Being located forward of the wrist and away from grasping hands, the Staple and ring also pose no hinderance for a leftie and therefore, may be completely ignored except as decoration. Unless the user is also assigned to an actual calvary unit equipped with horses (not very likely, although possible) in which case, the SRC would be quite handy to have. Therefore, should one be in possession of an example of the SRC ... Get A Horse !!!