pcm.aiptek {
		type a2dp
		bdaddr "00:0B:xx:xx:xx:xx"
	}

Assuming that /usr/lib/alsa-lib/libasound_module_pcm_a2dp.so exists this
works as it should. So we have PCM at the input side of this plugin and
we send SBC over AVDTP to our headphone.
 =20
      

i think this goes to the wrong direction.
1. Even if there exist alsa library for many programms it is sufficent
to use /dev/dsp and /dev/audio.
This mean that the soundcard driver should work independent of the
alsa library.
    

this is too short thinking. There are parts that don't belong inside the
kernel. And SBC encoding and decoding is one of them.
  

without the alsa lib
So why not be conesquent and put btsco int the snd-bt-sco module ?
Like other hardware the module is tooled via command line or IOCTL
wich bt-device it should handle.
    

b) There is an gerneral interface dsp/audio/mixer wich become crippled

Bluetooth is not only hardware. What you see is a dongle, but on top
there is a complete stack. We interface with L2CAP and this is a
software layer.