As of 1.10, message("foo") seems to no longer work inside instances.
This may not be entirely true. FTR, I tried it on a ZG run, and an MC run, multiple times throughout the encounter. I was in a raid both times. On all attempts, it resulted an error along the lines of "attempted to call global 'message' (a nil value)".
If I did this "outside" (Azeroth or Kalimdor), it worked fine, even if I formed a raid.