We ate our dinner in silence on the front porch. Erika was studying up on exhaust fan venting and I was brushing up on floor framing. Inside, we began by tacking nails in the sill in a 12" on center joist pattern so we could string up some lines and get a visual reference. This really helped visualize what obstructions we will have to overcome.
For every joist you interrupt, you need to double up the headers and trimmers. Simple concept. But the pattern multiplies quickly when you're trying to navigate around a diagonally-descending tub trap or a closet flange. This wouldn't be much of an issue if we didn't have to bear in mind the 9" between joist placement of our heating tubes down the road.
Erika sketched the layout and that helped us draw in some possibilities and get a better handle on the options. In the end, we ended up with a plan that basically boxes around the closet (toilet, commode, crapper, whatever) and the tub plumbing and relies on two stacks of cinderblocks to ensure good strength on either side of said box.
Didn't even break a sweat. But we did have to shower. Our place is the dustiest.
Addendum from Erika: Just as we were finishing up, I looked up and saw a fat mouse sauntering along the subfloor. It looked me brazenly in the eye and then ducked into the crawlspace. I'd never seen a mouse so completely unconcerned by the presence of humans. Of course, it may never have seen a person: the house has been sitting empty for over 2 years. It's a mouse house.