It doesn't meet all your criteria, but have you at least considered RCA #145? The barrel is almost identical to the one you have, and that gun is packed with interesting features -- the carving behind the cheekpiece practically glows in the dark. I need a smoothbore like another hole in my head, but I would love to build one like that. Anyway, it does sound like you are going in a different direction, but just in case you haven't made up your mind completely and haven't seen it, give it a look...
PS. The pictures for 145 in my copy of RCA vol. 2 are split between p. 610 and p. 616 -- I don't know if they are all like this or if I'm just lucky.