In article .com,
says...
My bath rim will be supported on a wooden batten fixed to the wall, to
avoid movement of the tile/bath seal etc. I want to come up with a way
of fixing the rim to the batten from below (i.e. under the bath), but
of course the body of the bath is in the way, so you cannot see or
access the area to screw / bolt etc. I don't want to fix the bath to
the wall using metal brackets, it has to be hard down to the batten.

Shape the batten so the rim of the bath hooks over it rather than just
resting on top, then lag it with silicone sealant before fitting the
bath. Fill the bath before the sealant goes off so there's a good
contact between bath and batten.