You could serve it with a little green salad and make a mustard vinaigrette for it -- that way even though the chicken itself won't have the same flavor profile, the dish in it's entirety will have that nice balance.

Call it pecan crusted chicken with maple glaze, and pat yourself on the back!

Don't think so - it will ruin the crispness of the outside.

I actually had the mustard - just literally forgot to put it in with the egg whites! Any chance I can just spread some on over the other ingredients?

Do you have any dry mustard in your cabinet? You could try mixing the dry mustard with some warm water and incorporate that with the other ingredients. Won't be exactly the same but will give you the mustard flavor