I have a pretty pat formula for LARP writing, when doing "one shots", things that aren't part of a campaign.
Give 'em some factions. Break people into pre-determined groups with pre-determined prejudices for or against each other.
give some people connections that go against those faction lines, like a brother/sister pair whose relationship is unknown. Add some other secrets, like love affairs.
Put the factions at odds with each other, add some in-game plot happenings and let 'em roll.
In MY opinion, the characters, their relationships and the dynamic between the groups should be so strong they don't really need you except to resolve combat and to handle player-generated curveballs. If you've written tightly enough, two GMs can handle 40 people for 4 - 6 hours, no problem.
--Av