[LazyPup]

I would definitely not permit anyone to drill holes in a tub.

If your house is over a basement, crawl space or if your tub is on an upper floor I would consider approaching the problem from underneath.

Begin by filling the tub with water so that the weight of the water will hold the tub in its proper place, then you can drill small holes about 1/2" diameter through the subfloor under the tub and inject foam through the holes.

Be careful in selecting the foam. Some foams maintain the same dimension as they cure while others continue to expand. I would insure the foam you use is the type that retains its dimension because expanding foam could lift the tub.