I had the same problem with a D-A 7800 front caliper and a thick fork on a bike that I built. Even the longest rivet nut was too short. I ended up making a custom long one out of two shorter ones, a stud, and a couple drops of loctite red (on the stud threads).
Easy to do if you've got a drill press and a hacksaw blade or a drill, vise, and hacksaw blade.
Bob