Well, I did check it, and found that I was mostly wrong. That is to say, my syllabus says - loosely translated:
"Two planes can call one another, at an appropriate frequency, or with the consent of the controller on an air/ground frequency."
So I was right that there does exist a well defined interpilot procedure, according to my text the messages must include the word "interpilot" and also the frequency; but I had forgotten (if I ever learned it properly) that it is subject to controller approval.
I am even more surprised that my text does mention the concept of "an air/ground frequency" which still sounds very queer to me.
I must admit I had it mostly wrong, sincere apologies!