In all brackets from 10 to 79 level, when one faction is ready to start a specific Battleground, everybody gets a global warning:
http://s24.postimg.org/uit10jp1h/Schermata_da_2014_12_26_15_13_46.png
http://s24.postimg.org/c72djeged/Schermata_da_2014_12_26_15_15_36.png
https://github.com/ShinDarth/TrinityCore/commit/b9a88638ca0037ec7a1e15aa604ffd2b817cefa4
Tested and working.