The standard "mint punch" seems to be concentrated mint tea, milk, ginger ale, and either a lemon or an orange--though lemon curdles milk, so that doesn't seem right. Maybe lemon extract. I'll have to experiment.
It seems like you could replace the spearmint with catnip, and maybe use half-and-half to appeal to a more "catty" base by making the beverage creamier. This seems like it would be a big hit at furry parties.
One problem, though--catnip has a strong edge of skunk (well, more like lantana, it's a bitter note at the edge of the mint flavor--more in the smell than the taste). I'm not sure what to do about that. Maybe sweetening with honey would help, but that's a cloying flavor--it would go well with the milk, but I'm not sure that it wouldn't overwhelm the delicate mint taste.
Maybe a few drops of cinnamon extract? Or strengthen the mixture with catnip essential oil? Hmm...