IRiver 'cradled' - how to get Rockbox in front of IRiver's firmware?

[Anvil2k12]

my IRiver H340 Rockbox is used as a stationary
device in a cradle which is switched on via my old
Pioneer amplifier's AC switched outlet.

if I am not holding the stop button
at the same time or take the IRiver
out prior to that, the internal firmware starts
charging, etc and has to be shut down.
tried without the battery - no change.

is there a way to make Rockbox be the only
or at least primary boot sequence in a cradle?

regards
Damian