Stewart, I certainly can't grow my plumeria in the ground, at least not during the fall and winter months, so this is just off the top of my head. I lived in NW Houston for 5 years, and I would think that you could grow plumeria in the ground there with some (minimal) winter protection. I don't think that protection would have to be much; a frame, some heavy-duty, clear plastic, and perhaps a bit of heat, maybe only a string of Christmas lights.
You'll want to rig up a way to anchor those temporary mini-greenhouses. Wind has a way of picking up temporary structures and tossing them, along with whatever is inside. Your ground temperature should not even be an issue.
Needless to say, because I have to keep my "trees" limited to 7' tall and 4' wide, I won't ever "train" a tree. I think a lot of their natural form will be controlled by genetics. It appears some trees are simply lanky and some are more compact. Perhaps you will have to judiciously trim some branches on those lanky-growers just to encourage further branching. In my experience, blooming tips encourages branches naturally, so just waiting for blooming may be all that's necessary. Folks that have their plants growing to tree-proportions and have several varieties, can give you more information on what to expect.
