Got a little help from CGPT so it might not be perfect, but this seems to work from my limited testing:

triggers:
  - trigger: state
    entity_id:
      - input_boolean.ups_power
conditions: []
actions:
  - choose:
      - conditions:
          - condition: state
            entity_id: input_boolean.ups_power
            state:
              - "on"
        sequence:
          - action: scene.create
            data:
              scene_id: light_states_backup
              snapshot_entities: |
                {{ states.light | map(attribute='entity_id') | list }}
      - conditions:
          - condition: state
            entity_id: input_boolean.ups_power
            state:
              - "off"
        sequence:
          - action: scene.turn_on
            target:
              entity_id: scene.light_states_backup
            data: {}
          - delay:
              hours: 0
              minutes: 0
              seconds: 10
              milliseconds: 0
          - action: scene.delete
            data:
              entity_id: scene.light_states_backup
mode: single

I've only tested it by toggling the UPS boolean manually and not actually cutting the power, so I'm probably going to need to add a delay, or a retry loop or something to make sure the scene applies consistently but so far so good! Thanks for the inspiration.