Hi Chelle. You could use a 100% shortening + icing sugar recipe (plus flavouring of course). There is a buttercream base product called So-Lite Tropical in cake shops here that you use instead of butter. Regular bakery/cake shops use it a lot. Or you can buy a product called Bakels Whip'n'ice or some such. It is meant to be temperature safe and can sit out of the fridge once whipped, for ages.
What I would do, however, because I hate the taste of shortening, and refuse to use that fake whipped icing crap on anything I make (check out the list of 'ingredients' on the side of the carton!), is to make my usual butter, icing sugar, flavouring, cream and chocolate buttercream (ie ganache mixed into a regular buttercream). Decorate as normal. Put the cuppies in a box and into the fridge to super-chill through before you leave. They shoudl be fine in an A/C car, and then on display for several hours before eating. I would not use my meringue-based buttercream for this occasion.