An overall problem with the whole thing is that [GM]s are almost never a visible force. I think that people would be much more careful of their actions if they saw a Judge Patrolling the streets, or walking through heavily populated areas. Now obviously this is not an entirley possible feat for you to operate so heres the suggestions.
Suggestion #1: At a randomly selected time Issue a [GM] to watch over the spawn of a popularly camped HNM, if a [GM] would simply watch the events of a Serket pop, a Fafnir/Nidhogg pop etc, they may be able to easily deduce rule breaking behavior made by those players present. Serket can be especially violent, with RMT running rampant and trying to MPK everyone around.
Suggestion #2: Have a single or small group of [GM]s who run a player based orginization on each server, this would allow you to darastically increase your manpower without needlessly increasing costs. Do not neccesarily give these volunteers and [GM] powers or abilities, however make them able to easily contact the head [GM] quickly to report any indescent and rule breaking activities. But do not make this system NPC based like a Mentor system where everyone can join. An applicant should be required to meet with the [GM] for their server in a one-on-one interview much like an application for a real job. To encourage activity possibly create a small in-game salary.
I beleive that Suggestion #2 would step up the level of authority that SE holds over Vana'Diel, perhaps even work the Judges(GMs) into the Story. If a tighter presence of authority is created, without becoming opressive, I beleive that illegal activites including MPK will be greatly decreased. As long as the players are not granted any major abilities or anything like that, but simply a special symbol or a special equipment that visualizes their connection to authority, and that player is carefully interviewed, and removed form service if they break any of their commitments, I beleive that it could become an effective way to increase presence without increasing your acctuall manpower.