Well it didn't work. The stuff still in the bag didn't set. It all disintegrated back into liquid form. There was a nice big ball of cured foam outside of the bag. It had continued to expand slowly, but nothing supporting the floor. I suppose it is possible that with the "blob" sealing the edge of the ziplock, there was nowhere for the foam in the bag to expand to, and then just retracted. I will do an experiment with the remaining stuff in the can and see if it can work and what the optimal volume into a ziplock bag might be
So I carefully detached and pulled out "The Blob" (turns out it didn't adhere too robustly to the floor), attached to the ziplock, and went back to the styrofoam block approach. Managed to cut a wedge of the correct size that would both fit through the gap in the wiring that I was working through, and slide in behind the vent pipe and still be large enough to support the corner. It needed to actually support the back wall and flat floor of the tub at that spot.
Floor is now nice and solid. I imagine with time and motion it may shift, but at least I know how to get at it now and exactly where I need to work. If at some point I can get the converter to shift out of the way so I'm not working through an arm sized gap between the converter and the wall, I might be able to do a more permanent fix. But until that time, I will get by fine with what's there now.