The current thread on AotH reminded me of that fun little detail about the casting of the Aegis at Tribunal.
It is done by attending Quaesitores, not by the members of the hosting Covenant.
And it lasts for a Year.
And as a ritual it is a pain to dispel.
So now you've got an "hostile" area of magic resistance smack in the middle of your Covenant (look at Durenmar, for example - I doubt the Tribunal's Aegis only covers the Forum of Hermes). Granted, the Quaesitores will likely hand back the casting tokens, but... they can still revoke them if they wanted to. And you can never be sure they did hand them all back to you.
Of course, in most cases, the covenant hosting the Tribunal is likely to be well-considered enough, and the Quaesitores ethical enough, that it won't be a problem. Tradition has an important role to play, too.
But you can easily make a white elephant of the honor of hosting the Tribunal. It can be great fun if you're hit with a Wizard's War, too.
Interesting, but this is all resolved if the Tribunal is hosted in a separate location than the Covenant.
It can be near, while not specifically on the same ground.
Having not thought of this before, I've always assumed that only rich (and prestigious, of course) covenants could host the tribunal because of the extraordinaries expenses.
Think about a noble welcoming the Royal court for some time.
I also assumed it was the case at Durenmar (were we've played many tribunal so far)
IMO, no covenant would tolerate such a situation as you describe. If they can obtain to host the Tribunal, they'll have a way to counter things like this.
It would indeed be the rare Covenant that would get excited about opening its actual doors and halls to any and all the magi of the Tribunal (or outside petitioners!), free to walk about at their whim. Possible, but a huge vulnerability to volunteer for.
In my saga, we actually lived for a year of gametime with a hostile AOTH on our (spring) covenant.
I had double-botched my casting of the ritual and avoided Twilight, so this was deemed an appropriate catastrophe by the troupe.
The magi shaked in fear and kept a low profile and delayed lab projects. As far as we know, no ennemies have taken advantage of the situation that we managed to keep quiet.