PX4: Forward MAVLink messages from TELEM1 <-> TELEM2

I have the following setup:

RPi APP "A" <= TELEM2 <= PX4 <=> TELEM1 <...RFD900...> MAVProxy <=> [MissionPlanner, APP "B"].

I would like to send a message from my software in ground station to my embedded software running in RPi via MAVLink. Using MAVProxy in the ground station, application B can send/receive data from PX4 but its custom MAVLink messages does not get forwarded to TELEM2, and I would like to send data to TELEM2 from application A in RPi and PX4 forwards it back to ground station so application B could receive response from sent commands.
Is there a way to do this without any modifications in PX4 firmware source code?