OK. I think I'm making some progress in my investigation. I'm aware that the Golf 7 system uses a MOST digital connection - this suggests that it may be possible to plug a Dynaudio amp into the MIB which, together with the appropriate coding changes, should allow retrofit of the Dynaudio pack.
Next step then. Can anyone with access to the VW parts catalogue post the schematic for the Golf 7 infotainment system so I can identify individual parts including the Dynaudio amp and the digital cables required?