Rear sights on fowlers are certainly very rare but not unheard of. Here is an original English export fowler from around 1790-1810 with an original rear sight (or at least an old one--it might have been added during the gun's working life):
While technically not a rear sight, many fowlers having a hooked breech had a hump just behind the barrel with a rounded recess and notch to facilitate aim.
I don't know if the latter solution would violate any smoothbore competition rules. Probably a local decision?
I also have an old 20 gauge fowler I made years ago which has the slot of the tang screw parallel to the bore and protruding slightly above the tang. (Not a hooked breech.) I have used this effectively as a rear sight.
Gregg