I think launchers have this interaction, already. And it's annoying, as the OP pointed out. When you're pressing ff repeatedly, and you suddenly have to press N, it costs a second or two. It would be much nice if the game simply said "Nope, can't do that", like it does for walking into deep water.
This should not be accessed though the same option as spells auto-target, because the player needs the option to throw Mephitic Cloud (or Fireball) in his own face. For Mephitic Cloud I also find it doesn't break the flow nearly as much, because I'll be expecting it.
I completely agree ff and f. can be preferable to TAB.
VeryAngryFelid: how many (new) players play versions where you need to choose which meat to eat ?
I agree about explore_auto_rest = false