Any gizmo that inspectors don't immediately recognize is gonna have permitting issues.
Your idea may work, though it's efficiency of transferring heat to the mass will take a BIG hit. Copper is expensive too and adding all the stuff adds levels of complexity and more things to fail.
Properly placed cleanouts make the swabbing out process a pretty small deal, 'specially since you don't have to do it very often.
BUT. For those that just hate doing the swabby job, or those who don't have the space (or underlying structure) for a good bench, there is an alternative.
Rocket
Bell Stoves incorporate a "bell" chamber, a masonry chamber of (relatively) substantial volume and surface area instead of the barrel and bench. Bells have superior heat distribution and can be made to wring just as much heat from the fire as long bench runs in much less space. They can be made to resemble Masonry Stoves (in fact can be made to look identical) in every aspect, giving inspectors an easy to recognize item to sign off on. They don't have the internal spaghetti of piping to deal with, the
ash tends to fall in predictable places and the works can be cleaned quite easily.
