I've had this "out-of-nowhere grabbing" my primary happen to me once, and fortunately I had the presence of mind to calmly comply (then take, clear and use the octopus for myself). But ever since that experience, I've made a point of making it crystal clear to any new dive buddy (before getting into the water), that I will donate my "yellow" octopus, not my primary. I typically dive with a 1.2-meter hose primary (routed under my right arm) and a 1-meter yellow octopus (routed under my left arm and attached clearly to my upper right shoulder strap).
John,
What is one of the major things that you and your buddy do before a dive? If you aren't doing a Pre-dive check with your buddy and going over the topics below then you're always going to have two divers who are expecting different things:
Planned Depth
Planned Time
Contingency Depth
Contingency Time
Buddy Signals
Emergency Procedures (lost buddy, out of air, etc)
So, with that said, if your weren't practicing this prior to the theft of your primary regulator I am in some way glad you learned a lesson to discuss these things with your buddy prior to the dive.
Now, to your question, is it acceptable to ask your OOA buddy to use your yellow-hosed octo? Sure, it is acceptable but don't be too disappointed if your insta-buddy panics and still rips your primary out of your mouth. Often times the mere discussion of what to do won't change what the diver first learned to do when in an OOA situation. In rec diving, as long as you and your buddy have an air source and you both are breathing there isn't much to bicker over.
In most cases your primary regulator is going to breath better than your secondary octo so if the diver is panicking. Who is going to need the better breathing regulator, you or your buddy who is in a panic?
All said, I don't see a problem with you asking a buddy to use your octo instead of ripping your teeth out for the primary but I also don't see a problem if the diver, in the heat of the moment, forgets that you asked that of them.