Is there any information available how the 3,5 mm AV-plug of N900 is wired?

I think that the most logical way to start would be to investigate if it is a hardware limitation or "just" missing software/drivers that prevents using of headsets with additional buttons like play/pause/stop/etc. If it is hardware limitation, then it makes sense to try to develop a way to use the one functional button of the default headset to have multiple uses, otherwise it would be worth trying to make the headsets with multiple buttons functional.