There are period references to its being done:
_A handbook for riflemen_, by William Duane, 1812
"The back woods men of the western frontier, place the ball in the palm of their left hand, and cupping the hand as much as possible, cover the ball with powder, and make that their charge."
_Audubon and His Journals_, Vol. II by Maria Audubon.
Describing a shooting contest in Kentucky, early 19th century:
"Each man cleans the interior of his tube, which is called wiping it, places a ball in the palm of his hand, pouring as much powder from his horn upon it as will cover it. This quantity is supposed to be sufficient for any distance within a hundred yards."
Describing a hunter loading his rifle before a raccoon hunt:
“He takes from his bag a bullet, pulls with his teeth the wooden stopper from his powder-horn, lays the ball in one hand, and with the other pours the powder upon it until it is just overtopped."
Sometimes they skipped the ball, just learned how big a pile of powder they needed.
_Anthropological Papers of the American Museum of Natural History, Hidatsa Eagle Trapping_, by Gilbert Livingston Wilson, 1928, interview of Wolf-chief 1915:
"A pin of hard June berry wood was used as a stopper for the powder horn. When loading the gun, I withdrew the pin with my teeth and poured the powder into the
palm of my hand."
Spence