The best thing to do is use the "Seller Info" profile to inject optional content to your templates. We have suggested to Auctiva that they make an "Options Profile" perhaps 10 tags, that vendors can place into templates.
This would be perfect for things just like your looking to do or other forms of optional content like a promotion such as save 25% or something for New Years Day etc.
Right now there is no options profile so we're using the seller info profile as well. In other words you could use the "[ABOUT_ME]" tag to inject your Xmas counter.