Steve, it sounds like no access panel was done originally, possibly because the tub faucet backs up to a hallway wall? If you have unfinished basement below the tub, you might look to see if the opening in the floor for the trap is large enough to do what needs to be. Otherwise, you may want to cut an access panel into the wall.
As to the foam & the possible problems Mike mentions, if your tub is cast iron you don't need to worry; the sheer weight of the tub will control the foam. If the tub is fiberglass or porcelain over steel, simply put about 4" of water in the tub before foaming & leave the water in the tub for several hours while the foams begins to set up.
If you access the tub from below, you might have someone sit on the side and see if you can see motion or pinpoint the squeak from below & this might help you direct the foam to the right area. Good luck