Ohh those are too cute!
I would take the e-motion border (http://www.wilton.com/technique/e-Motion) and pipe it in a tight circle. You can do it on wax paper, parchment paper or plastic wrap. Since I really like plastic page protectors I'd actually probably trace some circles on a piece of paper and slide that into the plastic page protector and tape that to a cake board. I use the page protectors cause it usually pops right off with out any problems.
I'm glad you posted this, because it is such a simple and easy idea, and would be great for quick holiday cupcakes. You could also pipe some trees too, if you wanted a little variaty.
I don't know how big your cake balls are, but I like to use the small/mini cookie cutters for outlines to fill in on cupcakes. 
i tried to do this but a little xmas tree, and it didnt hold its shape, I used antiona74's RI not thinned at all.... so next time should I make it thicker?
Add a little powder sugar or corn starch. I haven't used her icing so I don't know what the amounts are.
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