I don't know of any "documentation" but it's pretty well known an OOG diver very commonly goes for the reg they see in your mouth. Even if you have a perfectly good one held out towards them for donation. I was taught in all of my technical classes (when we discussed OOG long hose donation) to be sure to gain control of the person as best as you can and to be very cautious of the regulator in your mouth since they may try to rip it away from you. It's definitely not hearsay.
again just anecdotal hearsay ...
Every time I hear this "well known" cr@p I just roll my eyes.