Anything that's "oily", isn't going to dehydrate well.
For mac and cheese, I think I would go with partially cooking the pasta JUST before al dente and then dehydrating that.
I'd bring some suitable cheese, maybe the Baby Bell type wrapped in the wax covering.
Use powdered milk/with a dash of flour to make your sauce and grate cheese into the mix and blend at the campfire.
Then add the almost done dehydrated pasta and simmer until it's al dente.
I suppose you could rig a way to bake it over campfire as well.
Next trip I take, I'm bring some of that Laughing Cow cheese. They make some nice varieties.
I think you could make a "gourmet" version of traditional mac and cheese by substituting few wedges of the LC for the grated cheese.
Bonus of the LC, is that you can use it for a garnish to other food items or lunch spread.
I'm a big believer in experimenting and thinking outside the box especially when cooking.