Dennis.....
Very easy to get the rub under the skin. You can work your hand underneath the skin, gently lift up and separate from the body. You want to leave the skin intact, just gently lift it up. Once lifted, you can then sprinkle your rub down onto the breasts. When seasoned, simply let the skin fall back into place.
I do this for every chick or turkey I cook. A rub will NOT penetrate poultry skin, so you must make certain to season underneath the skin.