I would probably just leave it out. The mac won't "hold" as well, but with macaroni and cheese that doesn't bother me. You could probably add a bit more milk or cheese to replace any moisture you would be missing out on my omitting the egg.
I know that really isn't the type of answer you are looking for, but I have honestly forgotten the egg in things like meatloaf or casserole before and it did make a texture difference, but not enough to affect flavor or whether or not we ate it.
Re: Egg subsitute for baked mac and cheese
I would probably just leave it out. The mac won't "hold" as well, but with macaroni and cheese that doesn't bother me. You could probably add a bit more milk or cheese to replace any moisture you would be missing out on my omitting the egg.
I know that really isn't the type of answer you are looking for, but I have honestly forgotten the egg in things like meatloaf or casserole before and it did make a texture difference, but not enough to affect flavor or whether or not we ate it.
Hope that helped!