Assuming you have access, you could mix up some deck mud (say 5:1 sand to portland cement) and stuff it underneath. The stuff should be mixed so it is just wet enough to clump together when you grab a handfull and squeeze - it should not drip water when you do this. This is the mix that is used when building a shower base. It will pack down. Think wet beach sand, not runny concrete. A high density/low expansion foam that is designed to be flexible rather than get hard and crumble if compressed might work, but put too much, and you could crack the tub when it expands or cause it to bow when it lifts.