I have a trim little 16-gauge hammer double by V. Collette of Liege with 2 9/16 chambers that has had the barrels cut back to cyl/cyl, and got to thinking that it might make a fun round-ball gun with handloads in Magtech brass cases. Ordered a .650 round ball mould from Jeff Tanner for this purpose, and now I am looking at adding a simple rear sight. My thought is to file out a wedge-shaped piece of steel the width of the rib, solder same to the rib, and then file a shallow V after seeing where the barrels want to hit. This sound about right to those who have walked this trail before?
I'd rather not do anything more radical such as having a dovetail milled.

Sounds like you don't want to alter the appearance much; a peep with a big aperture would work nicely if it didn't offend your artistic sensibilities.

Tom, I'm trying to come up with a sight that doesn't require D&T. Agree a peep would be nice, but doubt this thing would be capable of taking advantage of the accuracy potential. I should probably shoot the thing first. Ordered a .650 round ball mould for it from Jeff Tanner in the UK.

Bill
I did something simular to an old double for slugs. I made a front sight that would fit over the rib at the muzzle and used a single screw to attach it to the rib in the dovetail under the sight. I made a simular to the front for the rear sight only used a ghost ring. It was quite accurate

If it's Belgian it should have continental thread. Why not try removing the top lever screw and replacing it with a sight that IS now the top lever screw?