If phlpp *wanted* to, he could program it in. There's already an animation for reloading a carbine in With Fire & Sword on horseback, and probably the same in other mods.
But I'd say that reloading a MUSKET on a moving horse would be incredibly difficult. You have to remember a musket is usually a considerably long gun in the first place, and reloading it fires the user to stand fully upright to put the powder and the bullet into the tube.
You can't stand fully upright on a horse - not in this game. Reloading the musket is impractical because it's just far too long to be practical. That's why carbines, shorter versions of muskets, were made - they were made for use on horseback.
So no, I wouldn't want MUSKETS to be reloadable on horseback, and I sincerely doubt it will happen
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Just an unrelated question, how common were pistols around this time period? I actually really want to pick one up for my next character, if that's historically viable