As an Engineer I agree with the assessment about the need to do two separate castings due to heat stress fracturing considerations. If you want the stove to last this is a must!
I have an idea how you can still ship a core and manifold in one, but you also need access to injection molding technology. The idea is to cast the heat heat riser out of high temperature refractory cement, then to set the cured riser into another form for injection molding a high temperature RTV Silicone spacer on the top and bottom surfaces of the heat riser near the outside edges.The heat riser would then be put into another form such that the RTV Silicone is slightly under compression and then cast a low temperature refractory cement for the manifold and feed box. The RTV Silicone will create a seal between the two castings, and will provide stress relief as the different materials expand and contract at different rates. Leaving room for sacrificial fire bricks (as was noted above in this thread) is a good idea.
Also, I have some ideas for a shippable rocket stove & mass if anyone want to work on it with me. All of the designs and bill of materials would be released under the creative commons attribution & share alike licence: http://creativecommons.org/licenses/by-sa/3.0/
(Everything below this line should be conidered to be under said cretative commons licence).
The basic idea would be to use sheet metal to create the outside form for the heat riser (with a burn out inside form). The sheet metal provides structural support after the fact. Sheet metal would be used to create the inside form for the feed and manifold parts of the core. The core would be set into a larger cast Aluminum structure that looks like that conceptual picture of the tall standing stove. Heat coming out of the manifold would enter a heat exchanger inside the lower part of the main volume of the stove. then exit out of a vent pipe near the top (This is then routed outside to exhaust). A separate heat exchanger is put at the top of the stove volume, it takes air from down near the floor and runs it through the top heat exchanger then out into the room (This should be a passive draw effect, no fan needed, it will be driven by the thermal gradient, though a fan could be optionally added). This whole unit should be light enough to ship, though bulky like a refrigerator. Once the unit is installed on site, the customer would fill the main volume with water (~200 gallons or so) to provide the thermal mass. There would be two other vents at the top that are opened directly to the internal volume of the heater to allow water vapor to escape (otherwise this is a hot water bomb, not a rocket mass heater). One vent would have a knob that you could turn to close it, and this vent vents directly to the room (for increasing humidity), the other vent would exhaust outside. It would also be nice to work out a way for the manifold lid to be external to the main volume so that it can be used as a stove surface. Concept drawings to come later...
A good design could probably be included into Open Source Ecology's "Global Village Construction Set" http://opensourceecology.org/wiki/Open_Source_Ecology
as well as be manufactured and sold by anyone.
Promoting my Company: http://opensourceecology.org/