For what it's worth I have been working on this extension for a while and have found a solution to have it automatically produce hats everywhere. You can see it in action
here. You'll notice that it not only produces a hat on the site logo, but also on the forum categories as well as two extensions I have installed on that site.
For other holidays to be considered, need some images preferably hat type ones (like new years, etc). I'll then consider modifying the code for an admin to be able to simply choose which holiday type they want to show.
The way it is coded at the site linked to above should fit all forum styles that use the default style tags (forumbg, forabg, etc).
All hail the power of jQuery!
EDIT: If you want to test out the version I have you can get it
here. I know Bill isn't able to do PR's at the moment so doing this here.