I havent read the rest of the posts properly, but as far as I know, bike helmets don't have the strength to hold up to the impact and can actually do more harm than good as the helmet can collapse into your head so I am told.
Snowboarding helmet? ehhh I would still be inclined to say no, just because they are designed for a completely different sport. they would probably be stronger, but I would still say go for a horse riding helmet just because they were designed for the purpose
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms