I know of only two guys who actually shot browns with muzzleloaders, and one bear guide who's a 50-year flinter on his own hunts. One of the two hunters used a 54 and the other used a 58. Both made good shots and needed all the extra "help" they could get from their guide after the shot. Their accounts make the bear shooting fiascos of Lewis and Clark sound pretty tame. One is going to try it again with a .75 caliber flinter he's building, but with two shooters for backup this time (one packing a 375 H&H and the other a 416 Remington).
The bear guide is as experienced and dedicated to flinters as anyone here and more than most. And he won't try it and won't guide anyone who wants to.
If I'm sure I'll see bears in any location, I either hunt elsewhere or stay home. I gotta say that on surprise encounters, my 62 feels kinda puny in my hands and I'm more intent on finding the backtrail out of the woods than confusing myself with ideas of "defense." Only way I'd think a 54 came close to a brown bear hunting gun would be with a 1:28 barrel and around 600 grains of bullet moving as fast as I could stand to push it. I bought one of those about 10 years ago with visions of bear rugs, then came to my senses and stood it in the corner of a closet.
Theory is fine, but I'm pretty sure you wouldn't find any well-experienced hunters or bear guides up here willing to back you up if you went looking for a brown bear with a 54.