I was playing in the joust this weekend at Days, and was going up against a Martell player with the Viper out. I was playing Stark and had Meera in shadows, and though I recalled the conclusion on the boards being that Meera could not blank character ability immune Viper, I thought it best to ask for a ruling. It was deemed by Nate (and later affirmed in a casual conversation with Damon) that Meera could blank the Viper. Helped me keep the Martell player at bay and get one of my two wins in the joust.
Please ignore if I incorrectly recalled the decision that was settled on here on the forums.