Yes. Kind of.
What I ended up doing was making my own backing brackets. I cut some steel flat bar to length and drilled/tapped 2x 1/4-20 holes in each one. I then fished some string through the top pre-drilled hole in the bed, let it drop out between the interior bed wall and the outer sheet metal, and tied it to one of the holes in my bracket. I could then pull the bracket up into position, use a small pick to align the bottom holes, and then secure it in place with a bolt. I could then remove the string from the top hole without the bracket moving or falling.
I used this method to mount a couple strips of L-track in the bed without using nutserts. It took a few hours, can be done with nothing more than a drill, hacksaw, and like $20 worth of hardware from Lowe's or Home Depot.
I don't have wheel well liners in the rear (yet), but I think you can probably do it with a little patience and a long hook to grab the string.