It looks like this has already been thought of on the inside, but the Nova API doesn't expose this option yet. The nova.compute.api controller defines the attach_volume method containing disk_bus and device_type arguments:

compute/api.py:

def attach_volume(self, context, instance, volume_id, device=None,
                  disk_bus=None, device_type=None, tag=None):

However, the HTTP API doesn't give you a choice and calls that method without disk_bus or device_type:

api/openstack/compute/volumes.py:

    try:
        device = self.compute_api.attach_volume(context, instance,
                                                volume_id, device, tag=tag)

So, to sum up, I'm 99% sure that right now you can't request other than default type of disk connection to the instance.