there's kind of a sneaky way to allow this, but I don't know if it works for gamepads.
Simply assign the same command to the same button.
For example, you have have ` activate "hotbar 2 button 2" and "hotbar 3 button 2" and stick the potion on one of those. I don't know how well this works, though, since I keep my potions on a separate key so I can time them better (being a bard, i don't have a super mega hard hitting attack like most classes do, so I have one running for setting DoTs. Which may or may not happen at the same time as throwing up buffs.
The way I see things, if they HAD a feature and removed it, there was probably a reason we're not being told. MIGHT have something to do with preventing quests from being macroed, but I have no idea.