That is not quite what I am saying, since that would mean that if you added another building in the middle of the covenant it would not be protected by the Aegis since it wasn't there when the Aegis was cast.
But if you just look at the area/volume protected by the Aegis rather than the specifics of what is in there then yeah, I suppose it would be implied by what I said.
What I am proposing is: For T:Boundary spells, the boundary can be moved if and only if that which it bounds moves with it.
I am thinking this should prevent your 'Aegis of assault' while allowing having an Aegis on a ship.