Curious....what will you do when a diver in panic grabs for it and can't get it off? If he/she is already at that point, I'm sure calmly pointing out that you have an octo available won't do so well. Seems like it's potential for an underwater fight over air.
This happened to me once -- If you design the slipknot right, it will slip off the mouthpiece with a big enough yank.
Incidentally, this guy wasn't out of air -- he jumped off the boat with his air off -- as in the OP's original comment. I turned his air on and he continued the dive on his own reg.