I don't see any need really to use mustard or oil to get rub to stick to meat. I don't use anything and I don't have a problem with rub falling off.
18.5" Weber Smokey Mountain
22.5" Weber OTG with Cajun Bandit Conversion Kit
Char-Griller Akorn with side cart
1999 Weber Master-Touch - bent but not broken
14.5" Weber Smokey Joe Silver