I'm in the process of building an indoor spa room. Even though I am not sinking the tub below floor level, I visited the drainage issue myself.
After some research, I came to the conclusion that for a floor drainage set up to work properly, one needs a 1/4" slope in the floor, similar to a shower stall.
Since the spa needs a level foundation to sit on, you would have to build a floor that is level in the middle, with the proper slope around the perimeter. Then have it slope and drain to a central point, where a sump pump or French drainage system (or your house plumbing system, if availible) would handle the overflow.
It can be done, but maybe it would be easier to just build a deck around the unit (building the vault covering on top of that)?
I once consider installing low openings in the wall/floor intersection, and using a squeegee to manually push the spilled water out. Instead I am going to have a wet vac, hidden in a base cabinet (vacuum garage, if you will), for the times when things get out of control.