The mustard is nuetral. Just makes the rub stick to the ribs. The mustard dissolves off and leaves no taste behind. If you use bbq sauce it will carmelize and most likely burn being in the smoker that long.
I have used mustard at times and other time I have not. Don't think I notice any difference.
What he said. I don't use mustard, just rub em dry. If it were me, I would not use any sauce at all, but offer the sauce on the side as an option for dipping the ribs into. Taste the ribs with the rub on them and if you or your guests think it need sauce, then you have it there for em.
If you want to use sauce, I would definitely wait till the end of the smoke to apply so I doesn't burn.
There are all sort of ways to get the rub to stay on. Mustard is my favorite but you can also use EVOO or Pam cooking spray as an example or no binder at all. The sugar in the sauce will definitly be a problem when cooking low and slow. I speak from expierence.
I've never had any trouble with my rub falling off. I did two butts last year side by side... one using mustard and I couldn't tell any difference in them. Get you some cheap kitchen throw away gloves and pat the rub into the ribs. It's not comin off. Just my opinion but the mustard was an unecessary step. Now mustard on chicken is a whole different story. Marinade some chicken in mustard and garlic and pepper and Worch Sauce overnight... now that's dandy.