I don't think I would do that. In my experience, water will likely get in anyway (as well as moisture in the form of condensation) and attempting to seal it up will cause the moisture to sit and eventually cause corrosion rather than drying out. As far as the spray in liner types go, you wouldn't have to deal with this particular issue, but I have heard that several of the spray in liners require that the original paint, primer, etc. be completely removed for proper adhesion, and I wouldn't want to remove all the factory corrosion protection. If you get a bubble or other problem with the spray-in liner and its just bare metal underneath, well... . I'm sure you can imagine what would happen over time. I'm sure not all spray in processes are the same, so if you go this route make sure you do your homework.
JMHO.
Dave