Ok, I tried my best to keep out of the fray till now. Just can't take it any more. Double check what before writing ? Your whimsical account of the noble and great Wyoming grizzly poachers ? A grizzlys skull is NOT bulletproof, it is just fractions of an inch of bone over the brain like any other skull. If a fellow had but one shot to stop a grizzly at spitting distance, coming right at him, a head shot would have as good or better as chance as ANY shot at instantly stopping the bear. Sure, things can go wrong, the brain is a rather small target in the skull, bullets or balls could deflect, etc. But, a shot anywhere else on a bear coming at you head on would have more things going against it for a one shot instant stop. If a bear is looking right at you, a shot right to the nose provides an easy pathway to the brain, right up the nasal and sinus cavity. Very little resistance. A bit lower, and the shot will go right in the mouth with a very good chance of doing some serious damage. If the bear has his head down, a shot to the top of the skull has a pretty good chance of hitting brain, or if back a bit, the spine. To STOP a bear instantly, a CNS hit is required. The brain is the largest target in the CNS.
Grizzlies are not the mythical bulletproof creatures some believe them to be, I have first hand knowledge of them being killed with .223s here in rural Alaska. A bit small for my tastes, but it has been done, sometimes with one shot. Many natives hunt them with 30-30s. They are nothing to trifle with, and can be extremely tenacious, but are not bulletproof.
So, the hypothetical hunter with his flintlock being charged by a grizzly? His chances are not the best, as anyone here would probably agree. He is already having a very bad day, with the odds stacked heavily against him. However, at ranges measured in feet or inches, a head shot would have as good or better of chance as any shot of instantly stopping the bear and preventing death or serious bodily harm. There are accounts of grizzlies dropped with a .22 lr to the head.
So put me squarely in the "shove the muzzle in the nose" camp, I would take my chances with that, regardless of the advice of the Wyoming bear poachers.