Route in-call audio to A2DP headset

How can I set up my Nexus 4 to receive Hangouts and phone call audio over a Bluetooth A2DP link? Even though my headphones are connected, in-call audio is routed to the earpiece or speaker and there's no option to send it over BT. I've got Creative WP-350 headphones that support both A2DP and HSP but the headset profile audio quality is unusably low, leaving no other option but to disable it in Android BT settings.