I'll do the same/similar thing... I'll rummage through my "foo foo gadget" drawer (sorry, couldn't resist) and pull out the flavor injector or any handy large bore syringe. I mix up softened unsalted butter with whatever rub i'm using (usually just a basic poultry blend for the sake of my 2,5 & 7 year olds) and inject it down between the skin and flesh as well.I like to separate the skin from the meat and put my rub inside the skin. Really helps trap the seasoning and lets the seasoning penetrate the meat deeper.
When that bird gets all roasting up in there you can see the rub and butter bubbling away under the skin... mmmmmmm I'm getting a little excited just picturing it! Lol