Don't neglect the personal aspect-- targetted asking people matters. It's time consuming and unpleasant for many people, but it makes a difference, and there are many GMs who will say yes if asked directly but won't volunteer or respond to a mass e-mail. Sending a message to a list is not the same as sending e-mail (or even better calling or asking in person) someone one-on-one, especially if you have a relationship with them.
I don't make it to many cons anymore, but the main factors that affect my con-going these days are: 1. relationship to the staff/to the con (this factor predominates); 2. quality of the gaming experience; and 3. location. My favorite con (
www.anonycon.com, coming up on Dec. 15-17 in CT, come play with us!) has terrific games, great GMs, and solid players, but the reason I go is because I care about the people who run it, even though I no longer live anywhere near it.
That said, if a con I didn't have a relationship with was trying to recruit me, quality of games would be the big factor for me. Do I trust the writers of the adventures I'm going to be running? I've run some really miserable mods, but I've also had an absolute blast running con games. If this is a big Arcanis event, are there "name authors" for the Arcanis campaign or setting that are writing for you? If so, highlight that. If there are things you can do to talk up the quality of your player base, that's also good, but that's harder to do in the abstract-- you mostly find out how good the players at a con are from how good they were last year. Obviously, targeting Arcanis fans will be a big draw for you-- "you love the campaign, you like the people who play it, this is the big thing."
Making it an "event" where there's something special or unique can also help, but that tends to help draw players more than GMs.
Finally, while I don't much care about this, many campaign GMs care a lot about slot 0s. If you can make slot 0s plentiful and well-organized, you can move people from the player to the GM column. (That's particularly important if you're asking your GMs to run the whole time-- 500 players to 84 GMs means every GM running, at least in the slot with peak player participation. Cons I've been associated with usually ask GMs to run 3-4 slots (out of 6-13), and are then super grateful to the stars who run more than that. If you're asking GMs to run every slot, you're both asking for a much bigger commitment and telling them that they can't play at your con.)
Hope this advice helps. Best of luck!