For years I used nothing but rub. Put as much rub on as will stick to the meat, wait a few minutes and it will get wet. If you want more rub, it will adhere to the dampened rub and more will stay on the meat.
Another option is to use Worcestershire sauce instead of mustard. You might even be able to use a BBQ sauce if its flavor compliments your rub's spices, but some BBQ sauces contain a small amount of mustard so check first.