Epoxy won't hurt the foam but I dont think what you're thinking of is acutally going to strengthen anything.

Get the lightest fiberglass cloth you can find and wrap the wing joint with one layer about an inch wide. Use an old credit card to squeege as much epoxy as possible out of the cloth and use 30 minute epoxy so it doesn't set up before you can get it all done. You'll get a very strong joint that doesnt add too much weight if you use the minimum glue possible.