Hi Wolfie
The addition of a on board Bluetooth device / system is something that has been suggested by several people here on the forum.
I also suggested the possibility of transmitting the audio via wifi, as I believe several of the home audio systems such as Sonos do.
As I have not tried fitting an ad on Bluetooth transmitter to the inside of a wallbox, I am unable to make judgement on its potential effectiveness when concealed in a metal box. It may have a detrimental affect?
When I have used a Bluetooth transmitter I have connected an earphone extension cable to the ear phone jack, this just protrudes through to the outside of the case, then connected the transmitter to the extension as and when it was required.
This way charging of the Bluetooth device is not a problem, although I would prefer it aesthetically if it were not visible.
Cheers Scudie