The mechanism is called "community managers who work for SE". I'll pass this along, they'll check if there's any foul play here.
Having said that, the proposal is still a long, long way away from becoming a site, so I wouldn't worry too much about it either way.
As for whether this has happened before... well. "This" is debatable, depending on what actually happened here, but yes, there have been proposals where folks have decided to create multiple accounts to either upvote or downvote questions en masse. We invalidate these as needed.
In the end, this sort of thing tends to be a self-correcting problem for new sites. Sure, a person or a group of people can create a bunch of sock-puppets to push the site into private beta... but then the site is pretty much guaranteed to fail since it doesn't have an actual community of interested users. We don't worry about it too much because of that. Trying to prevent a proposal from launching through shady means is a different matter, but it's pretty rare and generally quite obvious since it tends to happen only on particularly controversial topics.