Quote Originally Posted by kingcheesepants View Post
The problem isn't that you're losing the bag, it's that everything in the bag is scattered randomly throughout the astral plane when it's pierced. Unless you've got some really easy access to plane shift that makes the hide halfway inside a bag of holding plan a no go if you want to continue adventuring with everyone.
"Everything in the bag" in this case including the character's legs. Sure, go ahead and do that. [Evil DM grin]

I might let #1 or #3 happen, but I'd probably also allow the artificer to just build a waist-high well turret into his mount. But I'd definitely say anybody taking advantage of this gets no Dex modifier to their AC. For riding another player, I'd probably also say no melee attacks with any weapons bigger than a dagger, and no ranged weapons bigger than a hand crossbow. Your FOV would also be limited to whatever is BEHIND the character you are riding (unless the bag is in front, Baby Bjorn style), so you'll have some serious LOS issues.