I believe this should work, you can set up a specific volume type to create on a specific Cinder backend, then you can nova boot with the --block-device-mapping switch and specify the volume type.
The volume_type must be defined in the volume_type table in the database. When creating a volume for a particular backend (or group of > 1 backends) the volume_type is passed in. There can be > 1 backend per volume_type, and the capacity scheduler kicks in and keeps the backends of a particular volume_type even. In order to group a volume_type to a backend, you must define extra specs for the type.
cinder --os-username admin --os-tenant-name admin type-key lvm set volume_backend_name=LVM_iSCSI
then you can just nova boot with the --block-device-mapping option and make sure you set the specific type of volume.
--block-device-mapping <dev-name=mapping>
Block device mapping in the format <dev-
name>=<id>:<type>:<size(GB)>:<delete-on-
terminate>.
Reference Material:
https://wiki.openstack.org/wiki/Cinder-multi-backend
http://docs.openstack.org/openstack-ops/content/attach_block_storage.html