I'm in. I've been reworking some concepts on a novel that I've been meaning to work on and this seems like the perfect chance (seriously, I have a 10 page outline, compendium, etc I've just not had the drive/want the last year to start putting it all together). Any penalty to get a head start as long as you still do the total writing amount in the month? As in, I still plan to do the total amount of words per the month, but maybe start the first chapter or so.
There's no penalty, but I will frown at you!

Part of the fun is the camaraderie, the sense that we're all in it together. And part of that is the excitement of kicking off at the same time and going through the same things, the initial rush of excitement, supporting each other through the doldrums, that final second wind as we approach the end of the month and of our wordcounts.
You don't have to do it this way--as long as you're separating out what you've already written from what you do during the month, it's ok. But since you haven't started yet anyway, why not just hang on and start with the rest of us?