Nice set-up, Robin.
I've been watching this thread intently because I've flirted with the idea of some sort of highland grow space. I know how big these plants can get, and personally, I my desire would be to grow them to flowering size. This would require a fairly large space, say eight foot by eight foot by six feet tall. Not trying to grow villosa here, just get some nice sized plants like lowii, highland truncata, bongso, burbidgeae...you get the picture. I'm kinda in the same mind set as Griffin: look for a meat cooler as a source of cold air. Most are 240V electricity eaters, so I'm not so worried about cost of set-up, but cost of running the whole shabang.
Kirk, I know you're in Florida. If you plan on refrigerating a greenhouse, also plan on mammoth electric bills during the summer, because your giant cooling system will be running night and day!
Still pondering...
Trent