The principle of this hack is that I patch my aux signal to a free pin of the main signal selection IC on the audio PCB and hook a AVR microcontroller between this IC and the stereo's CPU which may then control the signal source selection and select my aux input when requested by a switch added to the front.

ADDENDUM: Unfortunately the ccb bus of this device is shared by multiple slaves, so main.c would have to be fixed to include address handling. I do not pursue this project further since I found a way to patch the aux signal into the line between the tone control IC and the output amp using the front headphone jack. Perhaps I will write down a bit about that somewhere.