who all uses peeps on their percussion guns.

[Muddly]

ALL my rifles are aperture sighted. Flint, sidelock percussion , unmentionable and even the ones needing suppositories. Aperture sights predate firearms ( crossbows). How much older do you need?
Fantastic sights. If modern appearing ones offend, then cobble up a field expedient or choose one of the vintage type repops. It is your rifle after all.