A clean-lean plugin

I am curious – what PHP did you have to change to get it working?

What php-fiddling:
Sorry, I was not clear; no changes in core code, we just had a series of “business-rules” (“strings” are persons playing viola or violin who have a status of “Aktiv”), and I added some filters following your example.

It would be nice to have an example with multiple keys, I could not yet get the following concept to work (but will, for sure).
But maybe it’s anyway better to have these in some cron-controled function.

$args = array(
			'meta_query' => array(
				0 => array(
			     'key' => $searchkey,
			     'value' => $val[$searchkey],
				 'compare' => '='
			    ),
				1 => array(
				 'key'   => 'status',
				 'value' => 'Aktiv',
				 'compare' => '='
		)));
        $uq = new WP_User_Query($args) ;

Just in case someone should need it:

function send_to_streicher()
{
    mailusers_register_group_custom_meta_filter('Aktive Streicher', 'streicher',1,"=");
}

function update_active_strings_meta_filter(){
  $newkey = 'streicher' ;
  $searchkey = 'instrument';
  $searchvalues = array(
        'Kontrabass','Cello','Violine I','Violine II','Viola'
   ) ;
  update_active_register_meta_filter($searchkey,$searchvalues,$newkey);
}

function update_active_register_meta_filter($searchkey,$searchvalues,$newkey)
{
    $uq = new WP_User_Query(array('meta_key' => $newkey)) ;

    foreach ($uq->get_results() as $u)
        delete_user_meta($u->ID, $newkey) ;
    foreach  ($searchvalues as $val)
    {
		$args = array(
			'meta_query' => array(
				0 => array(
			     'key' => $searchkey,
			     'value' => $val,
			    ),
				1 => array(
				 'key'   => 'status',
				 'value' => 'Aktiv'
				)
		));
        $uq = new WP_User_Query($args) ;

		foreach ($uq->get_results() as $u){
            update_user_meta($u->ID, $newkey, true) ;
		}
    }
}