making users.php search include a specific user meta data field without messing with the SQL query itself

Take a look at the pre_get_users hook. It is very similar to pre_get_posts if you are familiar with that, except it is for wp_user_query instead of wp_query.

You will need to ensure that you only check the users screen in the dashboard, because that hook would affect any user listing otherwise.

Here is an example of what the code might look like. Not fully tested but based on some working code from my own plugin.

function so339209_filter_users_indice_de_busqueda( $query ) {
    if ( !function_exists('get_current_screen') ) return;

    // Only apply on the Users screen in the dashboard
    $screen = get_current_screen();
    if ( !screen || screen->in !== 'users' ) return;

    $query->set('meta_key', 'indice de busqueda');
    $query->set('meta_value', $search_value);
    $query->set('meta_compare', 'REGEX');
}
add_action( 'pre_get_users', 'so339209_filter_users_indice_de_busqueda', 20 );