I think they call that a duplex load. I have often seen people on the range dump a little 4f down the bore of their flintlock and then load with their standard charge powder. Since I shoot only Goex in my flintlocks I have never seen the need.

I talked to a person that shoots a PA Pellet and they said to make the rifle fire faster, that is also the method they use. They dump a small amount of Goex down first, then seat the pellets. It give the pan spark something to ignite easierI guess...