Mike, you are getting close. There's two components to think about here, the burn chamber and the subsequent radiator/storage configuration. The only reason we can be fairly cavalier with the second component is if we know the burn is as good as it can be. So, I would say go with a tried and true front end that you know will not create creosote if you want to experiment with bells, or horizontal mass runs, or any of the wonderful options a rocket front end allows.
If I were doing this and hadn't done much experimenting with burn chamber construction I'd go with a tried and true. Frankly, the
Dragon Heater core is the only tested shippable core I know of with solid data on it's efficiency, and those numbers are fantastic. To my mind there's not much reason to keep looking to refine a J, Peter's done the heavy lifting here and Cindy is tackling the difficult task of producing them for a very reasonable price. If you are frugal and have skills and materials you can get awfully close using E&E's proven brick designs, or the various home casting methods available.
I posted a simple medium mass bell design on Donkey's board just the other day. It's basically the second drum you have drawn there with some bricks stacked inside for mass. Solves the sealing problem with just masonry and adds a little mass to a lightweight metal barrel bell.
http://donkey32.proboards.com/thread/904/simple-medium-mass-bells
So, if I were looking for a solution along these lines, I'd put a proven J-tube in a barrel in a classic
RMH configuration. Come out low from that first barrel, which is a riser/radiator combo, not a bell per se, and then bring the exhaust straight up to enter high on the side of the second barrel bell. Then exhaust down low from there and out of the house. I have basically this exact same setup in my bedroom right now for testing purposes, using my stand alone Walker Stove as the front end riser/radiator. Last night's test took the room from 59°F to over 80°F with one 4lb load and one 6lb load of dry fir. A little over an hour burn time and I had to shut it down. Room was still 68°F this morning after a night of 43°F outdoor overnight temps.