Braces that limit abduction external rotation are typically anchored to the body, so for snowboarding it really won't work. I've suffered 3 shoulder seperations, one from AER where my arm was extended above my head. I don't think there is really anything out there that will fully restrict your arm, but keeping it braced may help you remember to keep it tucked into your body. I've been eyeing this one. I saw Kevin Pierce wearing one similar to it a few years back at the US Open.
EVS Sports SB03 Shoulder Brace > Apparel > Protective Gear > Arm Armor | Jenson USA Online Bike Shop