I was taught to go negative, swim under or past and come up and control them by grabbing their first stage and lock legs on either side of their tank. They can't take your air source that way. I'd stay well clear of a panicked person's hands.
I'm not completely sure we're talking about the same scenario here, but if that's your response to a diver signaling OOG, I think you might be making a bad situation worse.
First of all, not donating as soon as possible would surely increase the panic of the OOG diver and escalate the situation. Secondly, if a panicked diver comes towards you for a regulator, and sees you head down, why would he not just follow you down, grab your tanks, grab a hose? I doubt he would just let you swim beneath him, essentially offering your back to him.
In my mind, although to be fair I haven't experienced having to donate, the best course of action is:
1. Dive with people who won't suddenly run out of gas
2. With good awareness you will see a situation develop and intervene before you reach the critical panic stage
3. If caught unaware by a distressed OOG diver, give them an easy gas source, donate a reg you know works (your primary) extended as far away from your body as possible, and as close to their face as possible. I would think they would go for the accessible reg first, not you. Hopefully quick access to gas will calm them down, you're in a position to monitor their state of mind, and can possible make evasive action if needed.