Perhaps I'm misunderstanding something that you want, but...
Call Control sample seems to transfer audio to bluetooth devices quite nicely? What are you trying to do?
Aha, I see your point now: yes, the WinRT API is a lot different - I haven't checked what devices are supported out of the box, and I must plead ignorance to how the "headset-alike" profile works on Win32 API with extensions.
You can however write Win8 style apps that use C++ libraries... Let me do a little digging.
On Windows 8, don't confuse the full .Net 4.5 Framework with the WinRT framework. WinRT is a subset of the full framework. So while you could possibly write an app that ran in Metro and accessed elements of the full framework, it would only run on x86 machines. As far as I've seen and discussed online, the WinRT framework does not provide any support for BT out of the box. I'm pretty sure you could write you're own stack, but that's probably not what you'd really want to do.
I am of the opinion that not's necessary to create a special windows 8 metro app. Only is need to create the metro app that needed, like Bluetooth control for the phone and the FM radio driver, and may be a navigation metro app, but the handling of music and other characteristics you can use the existing Metro apps.
In this video you can see an example:
The new 8.1 WinRT API includes bluetooth in the namespace, and new facilities for developers... http://msdn.microsoft.com/en-us/libr.../dn264587.aspx
Whether that exposes enough I simply don't know: rfcomm is there, but whether the audio streaming is there I don't know.