This would simply set the player as the destination and begin horses pathing towards that destination. This would be problematic in instances where horses have become stuck (i.e., fell down a ravine).

See the pathing branch and the following images for current difficulties:

I recall know that Tschipp's mod Callable Horse actually does this in a sane manner involving considerably less brute-forcing of AI and navigator. I need to look into it more because it should still be an option.