Interested!

Right now I have a cheap third party A2DP interface plugged into the AUX port on my empeg. The idea is for when my girlfriend wants to play stuff from her phone in the car. It works, but it doesn't allow for the buttons on the face of the player to send play/pause/skip commands to the phone.

An integrated solution (with onscreen track data and rewind/skip controls on the player) would be amazing, if you can get that working. I would buy one if it worked. I'd be willing to help you test the software, though I can't help you write it.

Would there be a way so that your device could have an AUX passthrough, to mix the actual AUX plug audio with your bluetooth audio, so that I could have my free AUX jack back? The idea would be that you'd resistance-mix the two sources, i.e., it wouldn't matter whether audio is coming in from the passthrough aux jack or from the bluetooth interface, either one or both will play sound. (I like being able to have my AUX jack available for a portable CD player for those super-rare situations when I buy a CD and want to hear it right away before I rip it.)

Regarding how to get the programming done: You'd need to interface with the kernel somehow. If you were already planning on having a connection to the serial port, you could submit a change to the hijack kernel which outputted some "coded" information on the serial port when buttons on the face of the player were pressed. Then your interface could read the serial port. Commands could be like simple ASCII text output on the serial port:
#E2DP:AUXMODE_SELECTED
#E2DP:LEFTPRESS_DOWN
#E2DP:LEFTPRESS_UP
#E2DP:AUXMODE_DESELECTED
You'd also want to do the same thing for commands from the Rio remote control.

Unless... Does hijack already have a secret setting for outputting button press information on the serial console? If so then no hacking needed for that part, you'd just activate that feature.

The Hijack kernel could also be hacked to interpret incoming coded data from your interface, containing track information, and put that up on the screen. That *would* need to be a hijack hack. But I'll bet it's doable.

Oh by the way, did you like how I already suggested a name for your product in my message samples up there above? "E2DP"? Cute, huh?