I'm mostly happy with my SL-9000, but this one
particular "misfeature" has driven me crazy from the
start. Does anyone know of an undocumented code or
trick for disabling Audio Overlay?

This is the mode that comes up when you press a mode
selector button twice-- "A_SAT", "A_TV" etc, which
causes some of the keys from AUDIO to bleed over into
the currently-selected map. It makes selecting
receiver sources with the input keys a lot more
tedious...