Universal Media Player - No volume_level attribute

Configuration
1

I’m using the Universal Media Player to automate a multi-zone amp that I’ve exposed over MQTT with a server I built. I have it working fairly well, but the media_player entity does not have a volume_level attribute and the volume slider in the UI is not reflecting the true volume. I can change the volume successfully with the volume slider, but it always defaults to zero and changes to the number entity I use to control the volume over MQTT do not reflect in the slider. What’s required to get the volume_level attribute so that it will sync with the slider in the UI? Here is my setup:

  - platform: universal
    name: Office Zone
    children:
      - media_player.whole_house_speaker
    commands:
      turn_on:
        service: switch.turn_on
        data:
          entity_id: switch.office_speakers
      turn_off:
        service: switch.turn_off
        data:
          entity_id: switch.office_speakers
      volume_up:
        service: number.set_value
        data_template:
          entity_id: number.office_volume
          value: '{{ (states.number.office_volume.state | int) + 2 | round(0) }}'
      volume_down:
        service: number.set_value
        data_template:
          entity_id: number.office_volume
          value: '{{ (states.number.office_volume.state | int) - 2 | round(0) }}'
      volume_set:
        service: number.set_value
        target: 
          entity_id: number.office_volume
        data:
          value: '{{ ((0 - (-80)) * (volume_level - 0) / (1 - 0)) + -80 }}'
      select_source:
        service: mqtt.publish
        data: 
          topic: devices/audio-zone/14/input/set
          payload_template: '{{ source }}'
    attributes:
      state: switch.office_speakers
      #Scale this to a percentage 0-1
      volume_level: >
        {{ (((1 - 0) * (states('number.office_volume')|default(-80)|int - (-80)) / (0 - (-80))) + 0) | float }}
      source_list: input_select.audio_sources|options
      source: sensor.office_speakers_input

Note that the MQTT server accepts the volume as dB gain from -80 to 0 so I have templates to scale the volume from 0…1.

Here’s the entity state:

source_list:
  - '00'
  - '01'
  - '02'
  - '03'
  - '04'
  - '05'
  - '06'
  - '07'
  - '08'
  - '09'
is_volume_muted: false
source: '09'
friendly_name: Office Zone
supported_features: 3460

And I’ve tested the template I’m using for volume_level and it’s working, returning the right value from 0…1.

2

From what I’ve seen in the code for UMP (after my volume_level template was ignored), is that the volume_level attribute is hard-coded to accept only one of ‘entity_id’ or ‘entity_id | attribute’ (the vertical bar is literal, used as a parsing separator, not a pipe).
So providing a template that returns a number makes no sense to the parser, thus it ignores it.
A feature-request might be needed here, to add something like ‘volume_level_template’ as an option in UMP’s config.