env-or-else with Ansible... possible?

playbook1.yml

---
- name: test jinja2 default filter
  hosts: localhost
  gather_facts: False
  tasks:
    - name: debug
      debug: msg="{{ lookup('env', 'UNDEFINED_THING')|default('foo') }}"

playbook1_output

$ ansible-playbook -i hosts playbook1.yml

PLAY [test jinja2 default filter] *********************************************

TASK: [debug] *****************************************************************
ok: [localhost] => {
    "msg": ""
}

PLAY RECAP ********************************************************************
localhost                  : ok=1    changed=0    unreachable=0    failed=0

playbook2.yml

---
- name: test jinja2 default filter
  hosts: localhost
  gather_facts: False
  tasks:
    - name: debug
      debug: msg="{{ undefined_variable|default('foo') }}"

playbook2_output

$ ansible-playbook -i hosts playbook2.yml

PLAY [test jinja2 default filter] *********************************************

TASK: [debug] *****************************************************************
ok: [localhost] => {
    "msg": "foo"
}

PLAY RECAP ********************************************************************
localhost                  : ok=1    changed=0    unreachable=0    failed=0

The solution, FYI:

---
- name: test jinja2 default filter
  hosts: localhost
  gather_facts: False
  tasks:
    - name: debug
      debug: msg="{{ lookup('env', 'UNDEFINED_VARIABLE') or 'foo' }}"

Thanks to Zart in #ansible for the help!