I believe they are very briefly mentioned in the Ultimania guide. Hell, if FFTactics Advance had not been released before XII, I doubt the Nu Muo would have been mentioned since I think there is a total of five in the entire game. One in Rabanastre, one at the Kiltia's temple, and then the group you run into when you undertake the sidequest to battle Humbaba, Fury, and Chaos.