You have a good question/thoughts about something that doesn't typically come up. At one time I wore boots (dress and work) all the time, in fact I've spent much time working horseback as well (cattle). In thinking about the situation you described I believe you typically aren't expecting to need some sort of quick draw rig, you'd just like to be armed while on horseback and have a secured weapon if need be. It's unlikely you're going to get "horse-jacked" and need a quick response. Personally speaking I would enjoy running my horse over a BG attempting to take it.
A couple of ideas on my part:
Ankle holster down inside your boot, I would think would be pretty secure and comfortable? That way the weight at the top of the boot won't matter. I think comfort could be an issue, while walking or riding you're going to feel the presence of that gun on every step. Think "rubbing".
Google "kangaroo carry", it's a clothlike band you wear under your shirt, and around your chest, that is somewhat like a shoulder holster. Pretty comfortable.
Saddle holster for a ranch rifle?
Nylon pocket holster, is fairly padded and I'd think be comfortable in your pocket. The one I use, would work while riding horseback (cheap Uncle Mikes and Ruger LCP). I never notice it in and out of vehicles or 4-wheeler. I don't ride anymore, but I can't foresee a problem riding with my little rig.