I have tested it again. AVRCP does not work with mediabox. It worked only with an
active mediaplayer-widget. But I removed that widget, because it is very buggy.
Now mediabox (mafw-backend) does not react to the bluetooth remote.
The only thing that works is pausing via headphoned, whtn bluetooth audio disconnects.
I think, it's best, when you catch the dbus-calls for mafw.