I know a little about SITA messages exchanged between airline offices and systems. They have a standard addressing format where :
CCC is the city or airport code
DD is the department code
YY is the airline code
For example JNBGGSA might be SAA's groups section in JNB.
Similarly, HDQRMAA is Sabre's main predesignated point for reservations traffic interchange, even though it's in DFW.
If the air<>ground codes are similarly formatted, I imagine that anyone who can send a SITA message to another airline would also be able to send one to an aircraft. However, I might be quite wrong, it's only a guess.