This may sound totally weird, but I grill roasts all the time, using indirect heat on the grill, and after rubbing the desired spices into the meat, I then completely coat the roast with yellow mustard.
Now, I HATE the taste of mustard, however when roasted with the mustard coating, I end up with no mustard taste at ALL on the meat, but it DOES have a beautiful char all the way around, and it looks like it just came out of a food magazine cover shot - every time.
And what I meant about indirect heat is that both for a gas grill and for a charcoal one, I have only half the grill heated (one burner on for the gas, and for charcoal I only pile it up on one half the grill). I then place the meat to be cooked on the NON-heated side, and rely on the heat circulating under the grill's cover to cook the roast. I end up with no burning - nothing catches on fire, and it is a wonderfully moist outcome, partly because of the indirect heat, and also because the mustard helps keep the spices adhered to the meat and it also seals in the moisture.
MMMMMMM, in any case - best of luck.
Bob Bone