It works!

conf.d__cinder.yml

storage_hosts:
  aio1:
    ip: 172.29.236.100
    container_vars:
      cinder_backends: "{{ cinder_backends_per_host.get(inventory_hostname) }}"

usage.yml

---
- hosts: aio1_aodh_container-c065ea73
  connection: local
  gather_facts: no
  tasks:
    - debug:
        msg: "{{ hostvars[inventory_hostname]['cinder_backends'] }}"

user_extra.yml

---
cinder_backends_per_host:
  aio1_aodh_container-c065ea73:
    limit_container_types: cinder_volume
    lvm:
      volume_group: cinder-volumes
      volume_driver: cinder.volume.drivers.lvm.LVMVolumeDriver3
      volume_backend_name: LVM_iSCSI
      iscsi_ip_address: "172.29.236.100"

Resulting output:

PLAY [aio1_aodh_container-c065ea73] ********************************************

TASK [debug] *******************************************************************
Thursday 09 March 2017  17:08:04 +0000 (0:00:00.024)       0:00:00.024 ********
ok: [aio1_aodh_container-c065ea73] => {
    "msg": {
        "limit_container_types": "cinder_volume",
        "lvm": {
            "iscsi_ip_address": "172.29.236.100",
            "volume_backend_name": "LVM_iSCSI",
            "volume_driver": "cinder.volume.drivers.lvm.LVMVolumeDriver3",
            "volume_group": "cinder-volumes"
        }
    }
}