For review only,

This patch checks that a device is already connected before trying to 
connect. If no device is connected, opening the alsa plugin will fail 
instead of forcing connection to default device (which is blocking).
This behavior can be enabled or disabled thanks to an option in .asoundrc.

pcm.bt {
    type bluetooth
    check_conn "yes"
}

It poorly tested now and mainly targeted for comments.

BR,
Frédéric