It's simple. the fat in the meat makes it juicy not the fat on it. the internal fat gets flavorful when it gets all warm and happy up there about 160 thru 200 degrees, The cap is not going to do anything but keep the smoke flavor and the rub from getting to the meat.
If you have direct heat your just flat out doing it wrong, all your doing by having a fat cap on the meat is filling the grease trap. I guess if you buy cheap meat (no one on CP eats anything less than Japanese Kobe) you could plug it to get the fat into the muscle but that's what us old schooler's call cheating. Injecting flavored oil into the meat. Not me.
__________________
|