With enough time, talent, and money thrown at it anything can be adapted to anything else. (
you want green laser sights on your dueling flintlock pistols, we can fix you right up if you got the bucks 
)
There are a wide range of grades of "vernier" sights and quality and price are pretty much parallel. There are cheap imported verniers that are way too sloppy and inconsistent for accuracy and very high end ones used for 1000 and 1200 yard backpowder shooting.
I think one needs to consider the way the rifle is intended to be used, out to 100 or 150 yards as a hunting rifle good open sight are probably most common and practical. I personally have a hard time expecting any patched roundball rifle to have a practical working range much beyond that--maybe 200 at a stretch--or 250 for an experienced expert on paper targets.
Tang mounted peeps like the Lyman and Marble sights do have practical applications at patched-roundball ranges ranges, especially for us guys who are getting age/visually challenged particularly on a gun for personal use---historical accuracy cops bedammed.
However verniers were basically used for long range and target sights and did not become common until VERY late in the patched-ball flintlock era, if at all. Not to say some one may have adapted one for that use way back when. In muzzle loading terms I associate them more with the precision long range ML rifles of the Rigby/Whitworth type in the post-Civi-War during the transition from L Percussion bullet rifles to the early cartridge singleshots.
In leafing quickly through the Hamilton/Rowe book "the American Percussion Schuetzen Rifle" I only note 2 or 3 of these very high grade 200 yard target rifles that used very simple vernier sights and one looks like its sight is a later addition of a cartridge rifle sight. Most of them use an adjustable barrel tang mounted lollipop or diopter
