I own several (3) RBL's and an A-10. The A-10 is the better of the two guns and didn't cost much more money: $3,995.
If you can make and sell a sidelock O/U for this and make money, I see no reason not to be able to produce an affordable Lefever.
Andy, of Andy's Custom Shop, told me several years ago that he owned the rights to the Lefever name. I know that he has fallen off the planet, but wouldn't you need this before any project started?