If an aegis is cast around a dungeon site before the exploration, things will go easier, right?
I would say such a casting is problematic on multiple levels.
First, it covers a boundary area. If that area is known and can be traced, then the spell can be cast. Second issue is less well defined, and I can't seem to find reference to it in the MRB, but I believe creatures within the Aegis at the time it is cast are not governed by the effects of the Aegis, but I'm claiming serf's parma, since I can't find a reference, even though I'm sure I've read it somewhere. Something that supports my view is the Black Fir in GotF, pages 52-53. Presuably the Black Fir gained freedom of action once within the Aegis, and has then been moving towards the Library ever since it has been inside the boundary of the Aegis..
I think it would work -- as long as the dungeon fell entirely within some "boundary".
Creatures within an Aegis at the time of casting are affected; it's just that if they are already inside, the effect preventing them entrance obviously does not apply (but the weaking of their supernatural powers certainly does).
However, keep in mind that an Aegis used in this manner is an attack. If a Hermetic magus is inside the boundary, it's certainly a violation of the code. If there are faeries inside the boundary, it's probably a violation of the code. And just about any being with supernatural associations within the boundary will be incredibly upset by an hostile Aegis coming up. So it's highly questionable whether this will make life easier for the would be explorers...
It also won't help with magic items (traps), since they would be inside the aegis at the time of the casting.
Casting a spell against a hermetic magus is NOT a violation of the code in itself. Turning the skin of a magus bright purple is not a violation of the code. Only certain castings against the magus are hermetic crimes. It could be argued that you are depriving him of his magical power but since the effect is not long lasting most of the time in quite a lot of cases you could get out of the mess with a fine. Also, unless the messing with fae and the like "brings ruin on your sodales", just annoying the hell out of them is not a violation of the code. 
This aegis casting is something we have never done. Nice one.
Xavi
I agree. However, a spell that, like a "foreign" Aegis, reduces a magus' ability to work magic, certainly constitutes a deprivation of magical power, and is thus a clear violation of the Hermetic Oath.
But it has a siede: it's limited to one Boundary, so it can evaded and the Aegis nor destroys the Vis Income, so it's not so breaking like Perdo Vim effects agains enchanted items or kill one Familiar.