I came across this thread last week when thinking of the possibility of modelling a crash helmet. It seems that this is a thing which is rarely done. I have posted some pictures of the helmet, and think it could be better i.e. taking out the waviness and ridge in the centre, but I dont want to spend the time tidying up what I can take away in photoshop.
I was trying to post the CAD file for you guys but it seems there is a 2MB limit, and the helmet is 3MB when zipped. It must be down to all those space points and datum curves.
Anyway, drop your e-mail on this thread if you want the file.