if you used 10 gallons of epoxy after spending 2 days cleaning all the rust off the bed it would be pretty much the same thing.

I see no issue with repairing a pan in this manner. The pan rusts out because it was drawn thin in the stamping process. If I had a soft spot in the bed because there was thin steel used I wouldn't hesitate to slather it up with fiberglass and call it a day, I doubt I would replace it with another bed that has the same defect from the factory.