Maybe I missed something here, and I realize the question was about the FAMAS, which IRL I know nothing about.
However, I do have a Competition HBAR AR-15, which has the option of a flat top receiver. However, I do have a sort of L-shaped rail which is affixed to the top of the carrying handle, via the hole in the middle of the carrying handle. It's hollow through the middle........then drops at the forward portion of the handle. On that forward end, I have a red-dot scope mounted. The way the rail is made and it's positioned, I can use either the red-dot or irons.
Of course a red-dot has no magnification, but I like using it in certain situations just for point shooting on the move and rapid target acquisition. For longer shots, I can either use them together, or just turn it off.
I just liked that idea, and would rather have the option of using irons, rather than mounting a scope on the flattop.
Sorry Jaunty, if I just basically said what you did in my explanation. I was gonna try to get a pic of it and post it...............