So why didn't I do this in the first place? Well, in its old spot the toilet was right between two joists and had a simple run for its drain that wouldn't have needed a hole in a joist. In this spot, I will probably have to drill at least one joist, and I don't know if I can put it there without placing the drain right over a joist anyway. It's also a tight spot for the shoulders, although it looks to me like it could work. You have to remember that the tub is low enough not to matter to the shoulders of a person sitting on the toilet; it just needs to leave leg room.
This room will have a forced-air vent so that the central air can reach it, but it's in the ceiling above the tub. If the floor heat works as well as they say, I may end up closing that vent off in the winter to let the other rooms have more air.
(Yes, it's that small. Yes, the tub is non-negotiable. But you'll notice I've got a nice little path built in . . . and the pocket door really helps.)