OK, I thought this a bit more and I think that I probably should clarify my thinking so that there is no misunderstandings.

As I see this, there are two things that are needed:
1. Button presses need to sensed by N900
2. Presses need to be translated to commands for Mediaplayer

For the requirement number 1:
As we now from dbus-monitor, in the normal state N900 does not sense the button presses from the headset. However, the call can be ended by the button press of the default headset. Now: what is awake and listening to the AV plug in this state? What awakes it (phone app or just incoming/outgoing call) How do we awake it without turning on the Phone app?

For the requirement number 2:
The headphoned also sends commands to Mediaplayer. I think that it would be relatively easy to expand the functionality to also send other commands. First we just need to find out what kind of signals different headsets send to N900. If I am correct, this could be probably found out with dbus-monitor when we first find out how to make the N900 aware of the button presses.

Does this make any sense to you more experienced users? Is there something that simply just cannot be done for some reason (technical limitation, I am suggesting wrong methods...).

After all, I am not a programmer, just a typical end user.