A2DP - How to send control commands TO sink device ( ESP32 )

[Hemingway]

I am using a Lyrat 4.3 with the Espressif IDF plugins for Eclipse IDE
I have started with the example project "player -> pipeline_a2dp_sink_stream"

I have the bluetooth module working with the codec outputting to a loudspeaker for playing music from my Android phone
The buttons on the board can control the play and pause
Volume up or down don't seem to be working but I'm not bothered with that

What I do want is to be able to control the codec by sending commands through a bluetooth app so the microcontroller can dynamically adjust settings such as individual channel gain
I have found the functions for button operation in the file "esp-adf\components\bluetooth_service\a2dp_stream.c" but I can only find the outgoing control and I want to add incoming control

Are there any passthrough command functions, or anything similar, already included somewhere which work in this way?