Sorry for the long hiatus, I'm back now if anyone has any questions. Only been a couple of years..
As far as I got with this project was having WinAmp run entirely off my laptop, with steering wheel controls and updating the car's screen via CAN (along with all track listings/playlists, sound options, EQ options, subwoofer volume etc). At that point to some degree the challenge/fun was over for me and I lost interest... but now due to ICE reasons I've decided I want to get it going ASAP, so going to take a little holiday from work and see where I can get.
Two main other things demotivated me: smoking my FM radio receiver and being unable to find any DAB+ usb receiver, and not being able to find a screen that'd fit neatly into the head unit I have. All a fraction too big!
I've now decided to simplify things though, run screen-less using just the OEM screen for info (why not!) and I'm no longer addicted to radio.. so I'll cross that bridge when I get to it.
So that pretty much means.. I'm already there, just need to hook a dedicated car pc up to the head unit's front panel (so I can use stock buttons/volume knob), rewrite it and probably open source what I've done.
Will update this thread as I go.