Ahoy Pauly,
This is more of a Suggestion, so I've moved it over here.
While "locked alliances" or something similar might be an interesting option, I'm guessing it would probably be very difficult to code it so a player can have the ability to assault certain players and not others for specified time durations.
One of the interesting things about alliances or diplomacy, in general, is that they are fragile beasts, and they may come back to bite you.

--Andy