I would suspect that there will be a way to retain all the functionality of the steering wheel controls with an aftermarket head unit, including the bluetooth phone controls.
It will be a case of ensuring you have the right head unit, and a suitable cable adaptor to feed said head unit with all the controls.
I am currently researching this myself as a decent aftermarket dvd/tv/nav unit would be nice, but also want to retain the functionality of the steering wheel as you do.
My Altea appears on the face of it to have EXACTLY the same software controling a bluetooth interface to my phone as the Z, even down to the voice. So I suspect there will be a manufacturer out there somewhere that has made the necessary harness to acheive this with aftermarket head units.