Interesting question indeed!
This PC / HC question has been around since long before the Bicentennial of 1976, although many feel that's where it got its big start...prior to that, there were plenty of other things to argue about, and argue we did.
In the late sixties and early seventies there was very little "readily available" reference material, and the owner of a book like The Cap Lock Rifle by Maj.Ned Roberts was considered the subject matter expert on Muzzleloaders.
Of course other books were available, but that required a trip to the Library and research....something buckskinners of the day had very little time for, so word of mouth became gospel, much like it would have back in 1820.
As far as the question itself, "would a modern day muzzle loader raise any eye brows?"......I don't think so, perhaps a bit of curiosity, especially from those with a small disposable income....would it shoot a little better, would it cost any more, and would it increase status real or imagined?
I think the question of how a little ol' percussion cap, something that had been around for only a few years and they had likely only heard of in 1820, how was this little jewel going to replace a recognized, time honored ignition system of flint & frizzen?
Now that would have raised more eyebrows than a coiled spring, shinny stock, or brass, at least in my mind.
I also think GH nailed it when he said there was NEED for a sturdy, and reasonable priced rifle....that "need" has never changed from day one, it existed then, even before "then", and it exists today.
Just one person's thoughts, and I'm sure we all have our own personal thoughts on this subject.
Uncle Russ