Steve,
I probably have no clue here but modern color CRTs seem to have their faceplates glued on to the bell with some type of Epoxy and those tubes get rebuilt, is this not suitable for the 15gp22?
Never having seen a 15GP22 in person I don't know what your up against.
In any case this is a facinating quest, I hope someone is going videotape it for a documentary
Eric