If you are that worried about resale value DO NOT START YOUR TRACTOR!
The 1700 hours I had on my last machine had a LOT more to do with my trade in value than the ROPS full of drilled and tapped holes (that the dealer happily took in trade and never even said a word about). Hours was the ONLY thing they mentioned, for it's age it was in great shape.
Use closed cell foam, it is waterproof and seals a metal surface (look up metal roofing and closed cell expanding foam) nothing to worry about as far as corrosion goes.
If any you are really that convinced that foam is going to change the integrity of the ROPS make sure you don't clamp anything to it either, clamping anything to it actually does create a weak spot at the clamp. If it is the boogeyman lawyer you are worried about this should keep you up at night.
I worry just enough about stupid litigation to purchase insurance. If it comes to lawyers, they can fight it out.