Can't speak from experience in the muzzleloader world -- still consider myself fairly new to it all. But from messing around with airguns for a while, I'm skeptical that any of those positions would work at longer ranges. Granted, it's not that they couldn't work. I just doubt that they would without considerable practice.
Spring-powered and gas-piston airguns are almost always shooting at sub sonic velocities. So there is plenty of movement that happens between the time that the trigger is pulled and the pellet leaves the barrel. As a result, they are extremely sensitive to hold positions. Groups can triple in size or more depending on where one's support hand grips the rifle or where your cheek rests and how firm a pressure your body touches the rifle. Benchrest? Only if you rest it on your hand in the exact same spot and rest your hand on the bench. Thousands of pellets downrange have confirmed this for me.
Smoke-poles don't shoot that slow but between the time that the trigger is pulled, the hammer falls, the powder is ignited and the ball makes it out of the muzzle, I doubt that the total time is much different from a spring airgun, possibly even slower. They don't have the 2-direction recoil forces of the airguns (back, hard forward then back again). But it's still a long time -- long enough for the rifle to move differently if held differently, which would cause the ball to strike differently. Even centerfire rifles show this when resting on a hard vs soft object. Rest that rifle on a hard piece of wood and you'll see the bullet strike a few inches or more higher than it would if rested on a backpack or other soft object. When the rifle is held differently, it will shoot differently. At shorter ranges, I doubt it would matter much. But longer ranges always magnify these effects.
Then there's the safety aspect of having the bore so close to your feet...
