"Did not meet requirements; missing migrations" error In a custom D6->D8 migration module, how does one selectively migrate specific content-types, vocabs and blocks, etc, without receiving the error: 'XXX did not meet requirements. Missing migrations XXX'  

I can already selective limit via migrate_prepare_row(), but anytime I do so content related to anything not migrated makes MI throw this error and fail.

Is there any core process plugin that addresses this type of scenario per row?
You don't describe precisely how you're doing your custom migration, but the key to missing requirements on migrations you are omitting is to remove the dependencies. The migration that is complaining will have in its configuration something like

migration_dependencies:
  required:
    - d6_user
    - d6_node_type
    - d6_node_settings
    - d6_filter_format
  optional:
    - d6_field_instance_widget_settings
    - d6_field_formatter_settings
    - d6_upload_field_instance


Simply remove references to migrations which you are not running.