I think that that type of sighting groove would be considered legal and accepted as it is not above the plane of the barrel. My smoothbore has a centre line engraved at the junction of the breech and tang. However, before you do this to your own gun, I must warn that it is so close to your eye that it is out of focus, and thus adds very little to the establishing of a reference mark. I am much better off to try to focus on the wedding band junction on the barrel, and place my front sight in the centre of that.
Regarding using a full round barrel, such as on a Brown Bess musket, I had good results with my Long Land Pattern 1st Model of 1728, simply placing the front sight at top dead centre of the breech. You're better off to let your eye adjust your sighting hold than to rely on some gimmick.