Example of adding an unregistered MU plugin to Loco Translate

mu-example.php

<?php
/**
 * MU plugins inside directories are not returned in `get_mu_plugins`.
 * This filter modifies the array obtained from Wordpress when Loco grabs it.
 * 
 * Note that this filter only runs once per script execution, because the value is cached.
 * Define the function *before* Loco Translate plugin is even included by WP.
 */
function add_unregistered_plugins_to_loco( array $plugins ){

    // we know the plugin by this handle, even if WordPress doesn't
    $handle = 'foo-bar/foo-bar.php';
    
    // fetch the plugin's meta data from the would-be plugin file
    $data = get_plugin_data( trailingslashit(WPMU_PLUGIN_DIR).$handle );
    
    // extra requirement of Loco - $handle must be resolvable to full path
    $data['basedir'] = WPMU_PLUGIN_DIR;
    
    // add to array and return back to Loco Translate
    $plugins[$handle] = $data;

    return $plugins;
}

add_filter('loco_plugins_data', 'add_unregistered_plugins_to_loco', 10, 1 );