Hmm. What if you left the mass raised in the center, but built raised wooden floors in other rooms to bring them to the same level? Lot of wood...
What are the downsides to leaving the floor level and using the entire footprint of the house as the mass? More insulation/digging..
'Theoretically this level of creeping Orwellian dynamics should ramp up our awareness, but what happens instead is that each alert becomes less and less effective because we're incredibly stupid.' - Jerry Holkins
The plan includes stairs down, so obviously some sort of basement. An underfloor plenum to distribute the heat would require the RMH core to be sunken to some degree, so I would suggest a batch box (so it doesn't need to be tended regularly) in the basement with a mass plenum bell possibly in circular form running around but not under the living room footprint... a donut of heat. The basement would need to be rather tall to accommodate this, or else the middle of the donut not used, but that would probably be the most functional application of the concept to heat the whole house. There are too many openings in the living room perimeter to have a bench along the walls.
A bell-type plenum would probably require minimum 4" of masonry on top, say a foot of height to allow stratification, and a few inches of insulating closure below to direct heat mostly upward. An 8' ceiling and a foot for regular floor framing would allow more than 7' of headroom under the plenum. Of course if you are not planning a full basement under the whole house, the setup becomes much simpler.
10 Podcast Review of the book Just Enough by Azby Brown