Give ground coffee a try in your next rub, Nathan. You may enjoy the subtle bitterness it gives the bark on a pork shoulder or butt. Coffee adds a depth and richness to other foods that I find pleasant. In tomato based sauces, I often add a half cup of leftover black coffee if available. If not, try a teaspoon of dry cocoa powder and a squeeze of anchovy paste from a tube into the sauce. All of these adds depth.
When ground coffee is added sparingly to your pork rub, you will hardly be able to single out that one particular ingredient. It works and blends in harmony with your other flavors for a very pleasing addition. Since you know coffee is bitter, also remember smoke is a bitter component as well, so don't overdo either.
Try adding a teaspoon of finely ground coffee in your next cup of pork rub. It's quite nice and goes well with the flavors in the bark.