Yes, this was the original rule that convinced me possession can fool the aegis in the first place. As I continued to ponder demonic possession it occurred to me that the original soul is also still a part of the body, and perhaps this is what is fooling the aegis. The original soul is not a part of the body in my story, so would the aegis be fooled? Yes, if the aegis targets the body, no if it targets the spirit.
I like this view. Meaning the aegis does not prevent mortal bodies and human souls from crossing it's border, i.e. the possessing magus can enter the covenant. However, their spellcasting will be effected. Could this mean that the aegis recognizes the sigils of spell casters? It makes sense. Those involved with the ritual, mystically come into contact with the aegis and it learns their sigils. Any foreign sigil will be resisted unless given a token which temporarily adds the foreign sigil to those the aegis recognizes. So my foreign wizard just needs to wait until the next casting of the aegis. Until then, hold up in your (stolen) sanctum and read for a year.
The inspiration for this whole idea comes from the antagonist of the adventure, The Traitor's Game, in TTT.
Cheers!