I want to start clicker training, but I haven't yet. But I DID teach both my horses to stand for mounting using treats. It works great. Now my friends do it too.
What I did was start out mounting in the usual way. Even if they walk off or whatever, just do the best you can, get on, try to make the horse stand, tap the side of their neck and give them a treat.
I use the tapping on the neck as a cue to turn and get the treat. But your horse won't know this at first, so you will probably have to turn their neck around using one rein while holding the treat out to them.
Anyway, after doing this several times they expect a treat as soon as you mount, so they really stand well for mounting. I didn't have a specific problem with mounting blocks with either of mine, but both would walk off and I am not that graceful and need a mounting block myself.
So basically just mount the best you can, tap horse's neck, give treat, and with luck they will start anticipating it and standing still for you to mount. It works great for me and my friends. Even one friend who was against feeding treats is now doing it.