Using root source property within migrate sub_process plugin

I'm performing migration from Drupal 9 into another Drupal 9 project. Let's say "content features" migration.

In a node migration I'm trying to create stubs for media items.

The media source has two id keys: mid, langcode. The mid key is getting from field's target_id property. But there is no language property on field's level and I'm trying to pass node's language as second id value for the media lookup.

The migrate_lookup plugin tries to find media entity by mid and langcode properties: MigrationLookup::transform():

if (isset($this->configuration['source_ids'][$lookup_migration_id])) {
  $lookup_value = array_values($row->getMultiple($this->configuration['source_ids'][$lookup_migration_id]));
}

then getMultiple() tries to get values from the current iteration.

Does someone know how to pass root row instead of iteration row?

...
process:
  langcode: langcode
  ...

  field_cover_image:
    - plugin: sub_process
      source: field_cover_image
      process:
        lang:
          plugin: default_value
          default_value: 'en'
        target_id:
          plugin: migration_lookup
          source: target_id
          migration: migrate_media_image
          source_ids:
            migrate_media_image:
              - target_id
              - ROOT_ROW_SOURCE_PROPERTY_LANGCODE

I wonder also if it possible to use constants within sub_process and $row->getMultiple()?

An example:

...
      process:
        target_id:
          plugin: migration_lookup
          source: target_id
          migration: migrate_media_image
          source_ids:
            migrate_media_image:
              - target_id
              - constants/default_language

I figured it out from sub_process plugin's documentation:

/**
 * Available configuration keys:
 *   ...
 *   - include_source: (optional) If TRUE, all source plugin configuration and
 *     values will be copied into the sub-processed row in a new property named
 *     for the source_key configuration value (see below). Defaults to FALSE.
 *   - source_key: (optional) If include_source is TRUE, this
 *     is the name of the property of the sub-processed row which will contain
 *     the source configuration and values. Ignored if include_source is
 *     FALSE. Defaults to 'source' if no value is provided.
 */

And used it as:

  field_cover_image:
    - plugin: sub_process
      source: field_cover_image
      include_source: true
      source_key: root
      process:
        target_id:
          plugin: migration_lookup
          source: target_id
          migration: migrate_media_image
          source_ids:
            migrate_media_image:
              - target_id
              - root/langcode # <- get property from the root row

And of cause constants also will be available:

migrate_media_image:
  - target_id
  - root/constants/default_language

An alternative could be creating iteration property as default value:

  plugin: sub_process
  source: field_cover_image
  process:
    lang:
      plugin: default_value
      default_value: 'en'

and refering to it as current iteration destination property @lang:

    lang:
      plugin: default_value
      default_value: 'en'
    target_id:
      plugin: migration_lookup
      source: target_id
      source_ids:
        migrate_media_image:
          - target_id
          - '@lang'

The content/media types set to be translatable but actually there is just one language and I can "hardcode" it, but it is better to use dynamic value.