Is there any concern that the hats, due to their time-limited nature, will encourage mass spamming of posts? Cosmopolitan, for example, can only be achieved by posting to Meta. There are over one million users on Stack Overflow who may want this hat, and have only until January 4th to get it, and their only means of doing so is to invent some reason to post to Meta.
So, this is covered in the Winterbash FAQ:
Hats? People will do anything to get a hat! It'll be anarchy! Hatarchy!
Whoa, hold up, no! All normal site rules still apply. If you get suspended, you're disqualified from hats for the duration of your suspension. If you're caught sockpuppeting for hats, you will be suspended, just as you would be normally.
Thus, normal usage of the site shouldn't stop. If you see things that are off-topic, that are duplicates, that are too localized, etc -- you should close them! In the same vein, if a user is spamming, you should flag for mod attention.
Winter Bash is meant to be a fun add-on to the sites to celebrate a great year. If users choose to abuse this event, they should be treated as they would if they were abusing any meta at any other time of the year.
This is meta! The fun begins here!
That being said, I'm quite sure you know how good meta(.SO) is at moderating itself. Most meta.X.SEs have a problem that they're ghost towns. A lot of people posting sensible stuff on a per-site-meta? Great! A lot of people posting crap? Doesn't matter, moderators get pinged every time a meta q is asked. We may get an overfull inbox, but we can deal with it. As long as we get a secret moderator-only hat. Please? :)
Can I suggest that any Meta post tagged with winterbash be excluded from triggering the Cosmopolitan hat?
There may need to be some interpretation of this, and for hats to be un-awarded should people not tag their meta post accordingly that retroactively get tagged with it, but hopefully that should prevent the mass-spamming of Meta sites.