i've tried this as well, and it didn't quite work the way i'd hoped. to get a nice sauce consistency i had to add so much it made the whole batch taste really, really tangy. i'd say replace the butter with yogurt and use some milk/whatever as well.
'course, that batch was one of the last i made huge batches and then refrigerated for leftovers. leftover mac just isn't as good, and with the tanginess and the fact that the macaroni were actually shells (macaroni is superior, even though the shells were also Annie's) it made the batch kinda bleh. It was decent enough fresh, though too much yogurt. Gonna have to find a nice balance, and see if the yogurt is compatible with the mustard.