I don't think using the foam is a good idea for two reasons:
1. You need the adhesive properties of epoxy to bond the fiberglass/foam/fiberglass back together where it has separated.
2. You're injecting that foam into a cavity with limited volume, so even the low expansion product is probably going to end up doing more damage that good.
If you want to try to find a creative solution, I would spend some time to really understand the structure, as well as the problem. Otherwise, just use a tried and trued method (Git-rot or West Systems) you can duplicate.
Same thing goes with the CompTip, depending on where the damage is, your repair may or may not withstand the forces that it will be subjected to. I'm all for DIY and backyard engineering, but safety is also important.