So Tim, your vote is for clay render to create an air barrier on the exterior face. Moisture seems like less of a concern for a material like wood-chip clay than for other materials, like straw bale or cellulose. I'm in the process of building a double wall to be infilled with cellulose, and debating the merits/necessity of an interior air barrier. I have osb sheathing (not my favorite material, but with typar, it kept the weather out for a year and a half while I got ready for the next phase of construction), so a little bit of care in lapping and taping the roof membrane (grace tri-flex) to the typar, and the exterior face is pretty much sealed. The inside will get sawn lath over 2x2 16 o.c., and clay/lime plaster. I'm puzzling over how and whether to bother trying to get a contiguous interior envelope, similar to the "air-tight drywall approach".

I like your greenhouse idea. I have a 2 story attached greenhouse in the works on my house if and when I ever finish the basics.
-Devin