good idea, I imagine that the idea has alot of possiblities. You could build a set of tracks, one on each side of the area and cut a 2 x to span the distance and mount a very long level to the area to make sure your level and then screed along.
you could then build a fence at the end of each track that would be functioning as a form or dam.
This all has merit. now again thinking out loud, when you mount the tup and the floor being flexible due to the nature of wood, when the tup is mounted and filled, do you thing that the floor will creek and moan and then break the thin set or concrete?
My thinking is to make it level with only wood forms. Then sit the tub on 1 inch marine treated plywood.