This isn't about radio communication, this is about exchange of data between individual vessels that is not contained in the standard VESSEL class.
As far as I can see so far, there is no way for an add-on developer to have different vessels that are designed to exchange advanced data with each other that is part of the specific vessel's sub-class.
Here's what I have in mind to remedy that situation:
A function like
char* oapiTagVessel(OBJHANDLE vessel, char* data)
could be implemented. Tagging a vessel would execute a clbkIsVesselTagged(char* data) for this vessel, in which the programmer could evaluate the string passed to it and define an answer (or execute instructions), which the oapiTagVessel function would return to be evaluated by the sender.
I chose a chararray because I think that would be most flexible, but it is well possible that there are better solutions.
This would enable add-on developers to easily create vessels that are designed to work with each other and exchange information between each other without having to abuse attachment points or other publicly available vessel attributes to construct a means of communication.