I have a buddy who is a lefty and has made a left handed safty for his MKII. It looks and works great. If he were to build a bach of these do you think the lefties of the world buy them? Or is someone already building one?
I cut a curved slot on the frame, opposite the safety. Threaded the backside of the safety button (6-40), notched the hammer, made an extention that passes thru previously mentioned slot and attached to the threaded hole. Ain't pretty, but it works. A pic is in the Ruger-Other gallery (Open Class 22/45).
There's a market for a left-handed safety that's well-made and reliable, but I don't think there's the sort of demand as you'll find for the ambi safety on a Colt 1911. I pretty much have to have the latter for a tactical firearm. I almost never leave my Ruger Mk II loaded with the safety on--I'm either shooting it, have it pointed downrange or it's unloaded. I sometimes forget the MkII even has a safety.
Still, I'd buy a lefty safety--the feature would probably get me to use it simply because it's more accessible--a control that's out of sight or hard to use falls into disuse.