Hey, glad to hear there's interest in this. I kept forgetting to check this thread.
Fellas, I'm designing this interface around a Roving Networks RN-52 Bluetooth module. They're about $22 from Mouser or DigiKey and don't actually require much other than some power supply components. The module sports stereo analog outputs, stereo microphone inputs, several GPIO for various uses, a UART for configuration and control, a PCM interface and i2s. It really is mostly self-contained, save for a few RS232 level matching components and a power supply. I'm also considering adding an AVR with appropriate code to emulate a tuner and allow for a steering wheel remote control without the need for a tuner, because those are made of unobtanium. Now that I think about it though, doesn't the tuner plug include an i2s interface and an RS232 UART? I wonder if I could just engineer a tuner emulator module and take advantage of functionality already within hihack. I'll read up the docs on the source and see what I can actually do with Hijack if I compile from scratch.
Almost all Android devices manufactured in the last few years will support both A2DP and AVRCP. I'm not sure about iThings because I haven't owned one since iPhone3G, but I don't see why not. They all should inherently support both profiles except for iPhone2G, but I doubt anybody is using one of them. I don't know if it's required, but the RN-52 module supports iAP mode for iOS devices with a custom firmware build from RN (Microchip). I don't really know what, exactly, this mode does or how it would affect use with Android devices. I don't plan on experimenting with that.
I'm not sure if I mentioned it or not, but this module supports HFP as well, and is capable of utilizing multiple profiles at once. I do plan on integrating that. Definitely should help to bring this "car radio" back into 2015.