cosmoc,
I dont think it is possible, as the altitude selection does not form part of the FMC(flight managment computer) on the aircraft. As I understand it, only the information from the FMC gets transmitted.. that is where the Flight ID, and route come from. The Transponder sends out the Mode S code which in turn gets registered on RB using the database to get the registration.
Only the cruising Altitude gets stored in the FMC for fuel planning.
hope this helps..