I think it was discussed before, and there's no chance wired remote control would work due to lack of hardware support.
But what you might try to do is connecting a bluetooth AVRCP device AND your headphones directly to N900 at the same time. I was using cheap BT-214 clone to switch songs while listening to music over FM transmitter. It was a bit tricky to set up, but worked flawlessly. So I guess you'd be able to listen to music over your headphones while connected to bluetooth (at least with some minor hacks or tweaks). Didn't try that, since just bought a good pair of BT headphones and forgot about wires.
BT-214 make a good remote control actually.