Marrakai,
The closest thing to your rear sight specifications (excluding a completly custom one) is, I think, as follows. On page 315, Brownells #58 catalog, there is the Track-Lock sight system. The upper half of the rear sight could be easily modified (flattening the bottom, cutting dovetails, thinning to fit the rib width etc). The screw hole could be tapped, and a new screw fitted, so you can adjust for windage and lock in place. Even the hole could be modified to fit your aiming needs. I do not have one of these sights on hand to tell for sure but some owner in the forum might help with dimensions. In Wilson Combat site, you can see the newer production of the same system. Maybe Mr. Wilson have some of the older production available and can also modify them to your specs.
Lefteris