The "in place" bathtub repair "enamel" isn't actually enamel but is just a chemical coating, I doubt it's heat safe. Real enamel is glass powder melted onto the metal at 1,500 ºF, similar to ceramic glaze.

However, there are places that -actually- restore enamel items by removing all of the old enamel and firing new enamel on in an industrial kiln.

There isn't any way to repair an existing enamel finish though.