The name you mustn't say is a common theme in many stories. I don't own every Ars Magica books, so i don't know if there is already something about this.
Anyway here is my shot :
Dire name (minor supernatural virtue)
You get an arcane connection to anyone saying your name outloud, this arcan connection last only for a few minutes.
You also get a score of 1 in the supernatural ability Dire name.
Dire name (Supernatural ability)
When someone say your name you make a Perception + Dire name against an ease factor depending of the distance between you and the speaker to notice it. This capacity must penetrate magic resistance.
A few miles - 6
Same country - 9
Another country - 12
Another continent - 15
Another Realm - 18
If your name is only wispered the ease factor is increased by +3, if it is shouted the ease factor is decreased by -3.
I'll chime in with Xavi here. I think your difficulties are a little low, but other than that, it's a lovely idea. Very mythic in feel, and there's a lot which can be done with it.
I can imagine a lineage with such an ability also having ranges and durations which work along side it, for instance. Duration:Name, which lasts as long as your name is clearly inscribed upon something - a little like circle duration, but impossible to hide your identity - something a magus of this type might not want to do.