Bobert...this is hard one.
I can only think that an epoxy type of paint/waterproof sealer would work!
Perhaps inquire with a Marina that refinishes Fiberglass Boats. Someone might be able to give you an idea about what they use!
If the tub encloser is enamel, well you've got me. That sounds a bit of a nightmare! Again I can only think of some kind of an epoxy Paint but getting the finish right would be...so much work..I think:)

These types of Paints, as you know, are generally pretty expensive and are labor intensive to use. Maybe just as cheap to have one of these Companies that fit a molded shell over your exsisting Tube encloser?

I'll be interested to see if anyone has tackled this themselves.
Best Wishes as always
Jude