You don't have to use a big pattern recognition, just look at the rotation angles of your camera and see how they change. define a threshold for the movement. let the player look at the question to have a reference and then see if you reach the threshold in both directions on the X or Y axis. at least that is how I would approach it - maybe I'll give it a try.
check out my Mobile VR Jam 2015 title Guns N' Dragons
I've done things like this, the biggest problem I encountered was false positives, but I was just looking in a direction, a nod is a good idea because its a specificly timed back and forth gesture. you could use the info from GetCameraPositionOrientation to analyze the movement and detect stuff like that.
edit: KatsuomiK, that's very cool of you, thank you :)