Seems to me like it should not be part of the boss module, then - it depends how often the buff would be available and what other things there are to prioritize that would be more useful to the raid as a whole, as opposed to just mages.
I can't know these things unless you discuss the encounter with me in detail.
A lot of the exact details of the encounter are still unknown to all but the most progressed guilds out there obviously, but what I learned from our night of tries is this:
Every minute or so (timer is already in bigwigs), 2 groups of 3 adds spawn from opposite sides of the room. These 2 groups consist of 2 hard hitting melee type mobs and 1 mage.
Regardless of the particular tactic a guild uses, these mobs need to be tanked and killed.
Now, there are a lot of choices a guild can make on how exactly they deal with these mobs since they are all crowd controlable. However it is generally acknowledged that the fight is extremely dps dependant and by extension tight on the number of healers.
The mage has a self buff that makes it's fireball attack instant cast and increases it's damage by 50%. For many reasons you do not want the mage to have this buff. First of all, your healers can't deal with 5k fireball spam at gcd rate on whatever tank you are using, together with all the other dmg that's going on. Secondly, if you can get your own mages to spellsteal the buff, instant cast pyro's at 50% increased dmg could help on the high dps requirement.
So, to recap, in order of priority:
1. The raid needs to know that the add has the buff so it can be removed in time before he rapes your tank. Removing the buff can be done by many classes so I don't consider this a mage only warning at all.
2. If a mage is in a position to spellsteal the buff and has both enough time and mana to keep this up, they will want to be informed whenever it comes up.
So, why put a warning in bigwigs for this instead of some other means of keeping track of it? Probably for the same reason we get warnings for pyroclasm on Eredar Twins, the fire shield in the maulgar fight and whatever buff the mage in the Illidari Counsel puts up. You tell me. While those buffs might seem more serious than this one and not just a buff on some add, you have to realise that in the M'uru encounter, in phase 1, the adds ARE the boss essentially, and keeping your tanks up against their dmg and dpsing them down is the hard part.
I just happened upon this topic, never really followed how bigwigs things are develloped, but as a user I would definately expect this buff to be part of the M'uru warnings.