There could be a health risk.
Personally, I'm against having any sort of polyester resin (aka fiberglass resin) based products anywhere near my head. Even with the most perfect mixture imaginable, small percentages of resin still remain unpaired with a catalyst and can release trace amounts of dangerous fumes months and and even years after it's cured. In a very enclosed helmet that's likely to be subjected to a lot of movement and temperature changes, I'm just far to paranoid for my health to ever consider this. I'd much rather just slushcast a few more layers of urethene inside it to thicken it up instead.
If you decide to do it, and for anyone who has, I'd definitely give it a good few thick layers of paint to seal it in.
