Personally, I don't like mixing techniques for fiberglass/resin with plastic or vinyl. Plastic is a very different animal. It doesn't sand very well. Some people would rather a fiberglass cast instead and do the dome work. That is exactly what I had: with the Golden Armor kit at:
http://thepropden.aokforums.com/goldena ... vt433.html
I did an eradication of the mounting system and a baldification of the skull using an epoxy clay called Aquamend which is a fiberglass repair compound. However, as much as Aquamend may stick to certain plastics, plastic will outflex it and eventually you will lose the weld. I wouldn't advise harming the plastic.
In fact, original condition Don Post Classic Action helmets are not easy to find nowadays.