I have seen the Santa hat, the reindeer antlers but is there a Frosty hat, complete with a corncob pipe and a button nose? If not, may I suggest one =)
The obvious answer is no, the hats that are displayed on the Winter Bash site are the only hats available.
However, there are Easter Egg hats (odd for Christmas, but, whatever), but it's not been verified what exactly they are (even to mods) or how to get them at the time of this post.
It's a possibility for next year, I assume, but the assumption is that the hats that you see are pretty much locked into place for this year.
There are, however, secret hats (as casperOne mentioned in his answer) which are awarded by surprise. Have fun finding them! Otherwise: we have no plans to add more hats to the base set at this time -- and I'll put this hat on the list for next year. :)