Rick-- A question..... In the aerospace industry we use ultrasonic testing to look for voids or delamination in composite structures. Flaps, ailerons, stabilizers and now, of course, wing and fuselage sections. This is nothing new---- we filmed Qantas maintenance engineers checking a composite flap surface for delamination using ultrasonic equipment back in the late 1990s. So would this not provide a pretty accurate way to determine the integrity of a fiberglass hull?
As you say it requires specialized equipment. In my 1990s example it was a two person job: one guy running the sensor over the surface and the other guy looking at a portable display. But wouldn't this same technology apply to fiberglass boats as well? Granted, it doesn't tell you anything about moisture content. But I would think it would show delamination and voids if they are present. And from what I've seen here and in Charleston the equipment is getting simpler and smaller and presumably less expensive all the time.