First off that sucks man and I hope you have as speedy recovery. I would not recommend it riding with a cast. But if it was my, I wouldn't give a shit if I had a neck brace. If it's winter and I am physically able to ride, I am. I don't care, every single opportunity I have to ride I am going to take, risk to my health be damned. I am not going to let anything stop me. If I die from it, that sucks I would rather die doing what I love than being 90 in a nursing home shitting my pants.
Sorry for the rant. All in all, not recommended, but you gotta figure out whats more important. Your health or snowboarding.
Im all for having fun, showing your tits and getting fucked up on a river float but it can be done without being a pig.