I made a patch, and drilled out a hole for the drain fitting. Here it is resting in place:
Something else I discovered: I'm not the first to work in this area of the van. There were small spots of black paint in this area of the floor, and they covered some surface rust. My guess is, someone discovered the rust, but not the source of it, and rather than cleaning it properly, they just painted over it with rust treatment. You can see the discoloration from the surface rust in the photos above.
Anyway, there's no sense in fixing one thing while leaving the rust undone, so I sanded and painted some other rust spots in the area with rust preventer, primed, and repainted the floor. Here's the floor, cleaned of all rust:
There's still some discoloration you can see, but these are stains, not rust, as far as I can tell. Then I treated all the bare metal with rust treatment. I didn't sand the entire floor to get a uniformly smooth surface, because this isn't visible: my aim is to prevent the spread of rust and to ensure body integrity. I did pay at least some attention to aesthetics, though, by painting it all the stock color so it wouldn't look too awful if somebody else came a long to work on it.
I epoxied the patch in place with JB Weld. Because the surface of the van isn't flush, and because I have a pretty small hole to pass a clamp through, I wasn't able to get a perfectly flush fit all the way around the patch. Frankly, maybe my patch is bigger than it needs to be, but I figure it does no harm. I did make sure the patch mates flush with the floor around the hole, though. The underside of the van was also slightly non-uniform, because of the two metal surfaces. I didn't want this to prevent a good flush mounting of the drain fitting, so I filled that with Bondo and sanded it smooth. Finally, I treated the underside with a rubber undercoating.
Here's the patched floor with the drain fitting in place:
Finally, the grand reveal: the cabinet is mounted back in place, with a new drain hose clamped to the fitting.
Time for a beer.