I swear by "Make it Fast, Cook it Slow" recipes by Stephane O'Dea. Her recipes for the crockpot are all gluten free. She has a Mac and Cheese recipe that looks good. You can find the recipe on her blog at crockpot365.blogspot.com or she has a couple of books.
I actually make our Mac and Cheese in a rice cooker but I have made many things in the crockpot with the rice noodles and they have turned out great. The noodles usually get added in the last hour.
Good luck and I would love to here how it turns out.