It is a sad thing, but, putting together all the pieces to pull off getting a bretelle Darne sling fitted to a 28 gauge Darne is extremely difficult from this point in time.
Contacts in France, or, someone who visits regularly, and has connections to the St. Etienne gun industry, and the ability to source new and used components, and fit it all up to the gun. Being a 28 gauge made this that much tougher.
I'm going to guess this won't be happening again, but, if you are thinking of doing the same, Geoffroy is your man. Fair warning, he might say no, depending on what you have.
You guys with a Darne 12, with the fitting in the stock, but, no sling, I'm going to suggest the time to try is right now.

Best,
Ted