Media keys are all already forwarded to the system, that includes play/pause, stop, next and previous. The only keys we might consume and thus not forward to system are volume up/down (check the setting in "settings" -> "navigation".
That means that, most likely, the key events your device emits are not standard android keys. We eat all those keys, because that's mandatory for us to be able to remap them.
Have a look at
https://github.com/koreader/koreader...customize-keys. The last link in the section will give you instructions to know which key events your device emits.
Hopefully you will be able to figure out the keys id (a number) emited by single presses on those buttons. Please report them here so we can forward them to the system. It is not possible to do it with remaps yourself if they're not standard because
that part of the logic is buried deep inside our input handler