Hello there,
as I understand it jolunda wants a rockboxed player to start playback at powering on and the player is powerwise somehow connected to the other devices. Rockbox has a feature, that a player stops playback when it is connected to a cigarette lighter in a car or to a power adapter when power is turned off (car adapter mode).
I was always wondering why the opposite thing isn't possible, that means to start the player when it receives power from the cigarette lighter. Then you could set it to resume playpack as start screen and using a modified .cfg file to always start playing back a specific file.
greetings,
c_k