It looks like there is a fair amount of space under the bottles. Could you use the straps that normally go around the rails of the saddle and thread them through the bottom of the cage holder? then still wrap the end velcro strap around the post?? Hard to tell what the bottom of the bottle cages look like or how much weight they would bear. You could even add extra length to the rail straps run them trough the rail, then secure then to the cage to stop any annoying swinging.

HTH