I update this thread to confirm I've installed RNS-E in my car with OEM Bluetooth module and left existing wiring for traditional Nokia 6310i phone in place, so both OEM Bluetooth and old OEM hands free system are now working with RNS-E together. This configuration is not mutually exclusive and it just works without any additional switch. RNS-E is coded for A6 (mute signal is taken from traditional mute pin and not from CAN bus message like in newer cars) and for OEM Bluetooth kit of course.
As you know, A8 have two microphone slots in overhead light. Traditional OEM phone kit occupies passender side slot (driver side in Germany), so second OEM Bluetooth microphone was installed in driver side slot. That slot was occupied by some electrical connectors for sunroof, etc., so I had to clean that space by rearranging those connectors a bit before actually fitting OEM bluetooth microphone there. Now both OEM microphone slots are occupied by OEM microphones -- fully loaded car!
You can even call from one phone to the other and talk to yourself. Thanks Ian for suggesting proper direction.
Now all I need is to find the way to take advantage of external antenna for my iPhone that I've connected via OEM Bluetooth
Organizing charging for iPhone is now the easy part.