A pan will buffer the heat hitting the meat. It will not, generally, speed up the cook unless the pan is covered, is of good quality or liquid is added to the pan or is allowed to collect in it. You can buffer the heat by placing the fatcap down on the cook grate and forgoing the pan, which many of us do.
You can speed up the cook by wrapping the brisket in foil at some point later in the cook--some do it when the brisket is just starting to plateau, some after it has plateaued for a while, some when it breaks plateau and has moved to or through 170--some never foil. Mnay base their decision to foil or not and when to foil on the specific hunk of meat they're cooking for that specific cook.
The juiciness we associate with tender, moist brisket (and many other meats) comes from the mostly interior soft fats and connective tissues that soften and gelatinize during cooking, not from the fatcap. The fatcap can protect meat (on the bottom by buffering heat, on the top by stemming evaporation) but does not do much to add moisture or fat to the meat.