Whatever your greenhouse shape, design and material used, the cheapest way that I can see to add raw thermal mass, is to insulate the perimeter to two times the depth of your frostline. If your frost line is 18 inches, then insulate to 36 inches. This turns the ground under your greenhouse into thermal mass.
Now, the idea is to HEAT that thermal mass, and I believe a rocket Mass stove can do that. (You may add additional thermal mass as you wish. Water as part of an enclosed system requires a very small wattage pump to circulate it. And it can be used to transfer heat from your heat source to your ground.) The problem is where to put the RMH?
The two main designs are 1) Full wall glazing. All the walls allow sunlight in. These work great if you get mostly skydome light. 2) South facing glazing. The northern wall is insulated (and maybe some or all of the east and/or western walls. These work the best for areas that have direct sun light, presumably from a low in the sky sun to the south. In the latter design, the heating system would go against the north facing wall.
The issue that I see with using a RMH in a greenhouse is that people don't live in greenhouses. There isn't going to be someone there to feed the standard RMH little sticks of wood all day long. So either you need to design some continuous fuel feeding system or come up with a way to have your fuel burn by itself for several hours at a time. MY idea is to do the latter, and to do it with a batch charcoal making stove.
The retort design for making batches of charcoal seems like the answer to me. You fill it with wood that you want to turn into charcoal, you surround that with some kindling to get the process started, and you stand back and let the thing go. The gasses that are released by the wood heated without oxygen exit the inner chamber and burn at a very high temperature when they hit air. They continue to off-gas for several hours. And when there is nothing left but carbon, the process stops. There are several videos of these stoves on youtube. I will not bother posting any in this thread. But check them out on your own time.
Now the trick is incorporating a charcoal making retort into a RMH design. This may not be straightforward. The batch method presents some logistical issues. you need to open the thing up and remove your inner drum, barrel or whatever, and allow it to cool before opening. One other issue, a charge of wood turning into charcoal puts off a LOT of heat. Seriously. You would need a fast heat exchanging system to cool all that exhaust fast enough for the gravity draw system of a RMH to work properly. AND you don't want to cook your plants in the process. Charcoal retort stoves tend to have a minimum size or else they will not off-gas long enough to be self sustaining. I think a 25-30 gallon inner drum is about as small as you should go. Smaller than that, and they will not put out enough off-gasses.