Don't do it. The reason why you wear full face on a MTN bike is that it is MUCH more likely to land on your head/face. I mean anything that sends you over the handlebars is likely going to be dangerous to your face. As for snowboarding, ESPECIALLY if you are not racing going high speeds down the fall line, you are way more likely to hurt a wrist. Broken wrist and tailbone are the most likely injuries in snowboarding. In skiing it is broken thumb and knee injuries. Head is lower on the list.
The nature of racing is that you don't need to be able to see your board or immediately in front of you as well as you would if you were going slower through the trees or just cruising. When you are going 60 mph, you are looking farther ahead than when you are doing 20.
Don't do it. If you usually ride Mach 10, then at least you won't look totally stupid wearing it. If you are just cruising... you will have to be replaying "Sticks and stones will hurt my bones but words will never hurt me" over and over in your head.