Where does the zipper break on tall boots? At the top, the ankle, the wide part of the calf?
The reason I ask is I have seen a number of pairs of tall fashion boots with a short zipper from the sole to just above the ankle on the inside. It seemed like that position would not interfere with your leg on the horse, not rub the saddle and also reduce the stress points on the zipper if they tend to break high on the zipper, plus it would give a really clean look when mounted.
I guess the downside is you would need a gusset to get your calf into the narrower top opening, but lots of boots have gussets even with the zippers.