Stepped wrist rifles turn up from Virginia down through Middle Tennessee from the late 18th century through about 1840. I would go with a flat toe instead of round if you do this style.
Just some general thoughts - I have seen a number of folky/backcountry fowlers restocked up in the 1790-1820 era from much earlier import/trade fowler parts and new late English flint locks. Granted these were not necessarily southern, but if I could make one general observation about them, the hardware and barrel were really all that was left of the English influence - the architecture and decoration of the restock reflected the style of the place and time they were restocked. (Again - these were back-country pieces - not pieces made in places like Charleston or Baltimore, where I would suspect a continuing heavier English influence.) The hardware was heavily reworked into the style and timeframe of the restocking - buttplates narrowed and shortened, triggerguard finials altered, a new folky sideplate made to match the new smaller lock, engraving filed off and partially covered over with file decoration, etc.
There are a lot of late 18th century references indicating that rifles were pretty widespread in the Carolina backcountry pretty early, so if it were me, if I was going for a back-country vibe I'd keep the architecture looking more "rifle-like" and keep the mounts more looking like imported, reworked or reused English mounts, if that helps....on the other hand if this is something that came from Charleston, Savannah, Baltimore, etc. maybe slip more English influence into it.
There are a couple of neat smoothbores in the archives - one was a probable North Carolina "smooth rifle" that Mr. No Gold posted, and the other is a neat Simon Lauck fowler that I believe is on Mark Elliot's website. Dennis has also posted photos of a nice iron mounted rifle that came out of the Roanoke Virginia area that has a musket style guard and big iron patchbox, that could work as a rifle or a fowler. I don't know the exact locations of these posts now, but these might be some good pieces to look at for ideas.
Another idea for decoration, etc. - look at some of the Bull pistols - these blend English duelling pistol influences wih backcounty materials and folky artistic flair. While they are pistols, not long guns, They convey sort of a feel of the times and place that might suit your project as well.
Guy