Have you seen xRobots method for plastic coating foam? I've given it a try and it shows promise, though you have to sacrifice some of the flexibility of the foam to take advantage of it, or at least be very conscience of it so you don't crack your butt plate when you sit down or something haha.
It basically involves foaming a piece, and then covering it in PVA glue (regular old elmers glue). Once it dries, you can put some smooth cast on it and slush the
outside of the piece. The plastic won't stick to the foam itself, but the PVA gives it a substrate to hook onto. Then you sand it down and can finish it like you would any other plastic part. I tried on a boot I was messing with, got mixed results. It's a TON of work.
I think hybrid foam/fiberglass is the way to go myself. Foam for pieces like the cod and boots, maybe the thighs so you can sit etc easily.
On my previous suit I did the plasti-dip method on my boots only- it does work pretty well, but you need liberal amounts of plasti-dip. It does have a little elasticity to it so if you bend a piece while wearing it you may not ruin it, but it might cause a seam or bend mark to appear in the finish.
Good luck! Loving that helmet too.
OH, Keep your eyes open for Volpin Props and his Needler (it is BEAUTIFUL)! He's said he wants people in Halo costumes to pose with it and your suit(s) would look snazzy