I would say yes you can use the foam to fill it in. I got plenty of junkers that were fixed that way. My mom (then brother) had an 84 Cougar where the trunk rotted in the spare tire well. My guy that does my inspections told me to just bondo over it. Once I removed the carpet, the hole was worse than we though, so I used window screen and bondo to cover it (it was supposed to be temp, but never got around to fixing it) That lasted for a couple of years. The last year it was on the road I used a whole tube of caulk to repatch the trunk area. Here in PA, as long as the holes are covers, they dont care. My father in law uses that Alum duct tape and then sprays primer on it. He has a 80's Bronco that has the all to familiar rot in the quarters.