Apple butter, never thought of that, good idea though.
I have heard of people that don't use anything except the rub, and still get good results.
The "slather" generally assists in keeping the rub on the meat, Typically I rinse the meat and then dry it off. Therefore, the rub will have a hard time adhereing to the meat. My home boss has a more sensitive palate than mine, and she doesn't like mustard. However, she has never noticed the mustard I put on the pork butt she LOVES. I also add that she is not a big fan of pork.
I have experimented using syrup (too runny), molasses (burned slightly(my fault)), and other liquids. Oil worked well, just a light coat.
I have actually used the THICK Le*a & P*errins Worchester Sauce and then rub and it worked fine. I use it on beef a lot and it is great.
If you have time, place the rub on the meat and then wrap in plastic and place it in the refrigerator overnight, or even just a couple of hours.
The moisture in the meat combines with the rub to form a sort of paste that will adhere to the meat better than the rub alone.
Either way, good luck.