Marinating, brining and preseasoning are the best ways to really get some flavor into a piece of meat, but seasoning on the grill while cooking is perfectly acceptable. This will give you another layer of flavors, the seasoning on the outside will be the first taste someone gets while eating. I do this when cooking steaks, we have a local steak seasoning that I love, so I preseason and also sprinkle a bit on mid way through the cook. Personal preference, so season away!!