tbh, I dont know if anything is public or not as i've personally never got to play with one of these boxes but disabling talkback shouldn't be too difficult. All of these boxes have a similar circuit for talkback which usually resides in the cable interface module. It usually consists of a return path amplifier chip and an impedance matching transformer.
The trick is to find the RPA chip, google for its datasheet and then take the appropriate action to disable its function.
If you spot a chip that looks likely then google its details. It should become rapidly apparent when you get the correct chip !