the project has run up a big engineering bill, to be sure.
the chimney is being built just as you suggest, John, block core with some rebar and stone facing(there will be a euro-style heater in the masonry work as well).
The chimney rings are 5' outside diameter for the second floor and 4' outside diameter for the roof ring. The chimney footing is over 8' diameter....
the lateral support is being provided by the strawbale wall system.... our timberframe includes radial bracing and the site-built porch frame has circumfral bracing, but both these bracing systems will be redundant once the strawbale walls are finished.