A task to emulate `-e foo=bar` in a playbook

set_global_var.yaml

---
- name: "Set a global variable mid-playbook run"
  run_once: true
  set_fact:
    "{{ global_name }}": "{{ global_value }}"
  delegate_to: "{{ _all__hostname }}"
  loop: "{{ groups['all'] }}"
  loop_control:
    loop_var: "_all__hostname"
  

some_playbook.yaml

---
- hosts: some_limited_group
  tasks:
    - name: "Set a global foo=bar for all hosts, not just some_limited_group"
      include_tasks: "set_global_var.yaml"
      vars:
        global_name: foo
        global_value: bar

why_you_want_this.yaml

---
- hosts: some_limited_group
  tasks:
     # Let's say we have a task to rename hosts and it sets a fact of new_hostname on the host
     - include_tasks: "change_host_name.yaml"
       
     # Record this group globally
     - include_tasks: "set_global_var.yaml"
       vars:
          global_name: "target_hosts"
          global_value: "{% set colon = joiner(':') %}{% for host in play_hosts %}{{ colon() }}{{ hostvars[host].new_hostname }}{% endfor %}"

- hosts: all
  tasks:
    - name: "Refresh the inventory to reset all host groups and pick up the hosts at the new hostname"
      meta: refresh_inventory
    # Host variables are preserved!
    - name: "recreate the some_limited_group with the new hosts"
      add_host:
        group: "some_limited_group"
        name: "{{ item }}"
      with_inventory_hostnames: "{{ taget_hosts }}"
      changed_when: false
      
- hosts: some_limited_group
  tasks:
    - debug: msg="{{ inventory_hostname }} is in the group."