I believe that that kind of slide-out underlug is occasionally found on rifles made in the Valley of Virginia - IIRC, Pete Alexander mentions them in connection with the Sheetz family in his book on building longrifles. My copy is in storage and inaccessible at the moment, so I can't look it up. I don't know if the method was used elsewhere or not, or how common it was in VA, but that might be a starting point for you.
I like the idea too, as it would seem to combine the advantages of barrel keys without the added complications of building and the risk of losing the keys. However, it could only be used with straight or near-straight barrels and possibly only with certain shapes of barrel tang, and I suspect that it would work best with barrels that had the rear-most barrel lug out in front of the entry pipe. That way, once the screw through the breechplug tang was removed, the forestock could bend slightly and let the tang rise up out of the stock enough to let it slide forward. With a lug in the more rigid area behind the entry pipe, the breechplug tang mortise would have to be undercut considerably to allow the barrel to slide forward enough.
Simple enough in concept, but possibly rather fiddly to get to work well in practice.
