Certainly, you can go that way but I think I'm going to experiment with them being like the character's shield, armor or weapons. I've never once seen someone declare "I'm attacking his shield!", and beyond effects like heat metal or rust monsters (who have a special attack), pretty much the same for armor. Now, you can disarm someone of a weapon, like you could possibly dismount someone from a mount, but its relatively rare it happens. Adjusting the rules the characters focus primarily on the rider, not the mount, I think is a feasible state to get player's minds into.
More difficult will be the "drop them off at the dungeon entrance", but that's kinda like parking a car - the players should generally expect to be coming back out near where they exited in most cases. If not, might consider modifying things so it's possible to bring the mount. As a last resort, I suppose magic could be invoked, but that's really my last resort.
