You bring up a point... These boots have metatarsal guards in them. Meaning they don't flex nearly as much as normal boots do. In theory, truck bed liner is just tough rubber, so as long as it stays adhered to the leather, it should flex with the boot, it just may not flex as much.
Worthy trade off IMO if my feet stay dry.
I forgot to add. Before I taped the boots off, I scuffed the leather stitching and sole up with a wire brush to promote adhesion.