Although I don't like to see the screw-on aperture sights on muzzleloaders, there were integral aperture sights on some Matchlocks, with a hole bored through the vertical protrusion at the breech acting as one. Some original, made in the USA rifles like Hawkens, had a rear 'horned' sight without any central knotch fro the blade, wherein sighting was or could be taken as with an aperture sight, in the middle of the 'hole'.
Such sights would fine and OK as they are historic. The problem is, from a rule standpoint, allow one closed peep, even a tiny lump of dirlled raised metal and someone will show up with a Central, Redfield or AJ Parker rear sight claiming it to be identical as to rules allowance to the original one on your rifle. A small diameter buckhorn can easily act as a peep, without hunrting rules of one's sensibilities. Of course, where or how close you mount your buckhorn sight to the breech is up to you, but I'd at least make it look like an original open sight.