I've seen dovetails/grooves milled into the side if the rib and 11mm rings used, my Bernardelli combo gun is done that way, it leaves a clean rib when the scope is off.
Recnagle(spelling?) makes turn in mounts for double guns and there are always claw mounts.
You can get quite thin steel Weaver style bases and mill them into the rib just as long as the feet of the rings clear the barrels, depending on the height of your open sights they might be able to attach them to the rib without milling.
http://www.weaveroptics.com/rings_bases/bases/steel_top_mount/ You can do just about anything, as my old boss used to say "We do the difficult, the impossible just costs more"
I hunt with drillings/combos, the same problems exist except the rib needs to be clean to use the shotgun side of things, even the rear sights flip up out of the rib or tang and the front sight is much like the front beed on a shotgun.
