Hi Ken - the Joseph Bogle rifle has iron keys - very small rectangular ones in cross section. There are a few other iron mounted guns I recall seeing with keys - like the William Cole rifle, that look to be of SW Virginia origin.
I will have to check - I think I have also seen a picture of a Bull gun with keys. The "John Bull for Isaac Guess" rifle looks to have escutcheons, but from the picture I have it looks like they are for pins rather than keys.
As far as the classic east Tennessee, Unicoi/ Bean or Lawing style gun, I can't say that I've seen keys used. Maybe because those forends were just so slender and sometimes get almost vee-shaped down through the forend, keys might not look right with that style (?). But given the use of them on some of the earlier ones, like the Bogle and Bull guns, it could be a neat build, especially on an early gun.
Guy