Change input term when connecting one post to another

I understand Scribu is no longer supporting ‘Posts 2 Posts’.
I’m asking someone who has altered the core in order to meet their needs.

So far I’ve been going through files item-user.php, side-user.php and query-user.php but I’m struggling to find the code I need to alter in order to search the users by nicename or first_name instead of user_login.

Any light on the subject would be greatly appreaciated.

Thank you.

By altering the constructor in file side-user.php I’ve managed to somewhat accomplish what I wanted, but not quite.
Using search_columns I can define what fields to be searched on, however I cannot use display_name or first_name since that requires a meta_query.

function __construct( $query_vars ) {
	$this->query_vars = $query_vars;
        $this->query_vars['orderby'] = 'display_name';
        $this->query_vars['order'] = 'ASC';
        $this->query_vars['search_columns'] = array('ID', 'user_login', 'user_email', 'user_nicename');
    }

I know how meta queries work, however I need to define the value of the meta_key and I have been unsuccessfull so far as to retrieve the content from the input field.

This is an example of what I’ve tried so far:

$this->query_vars['meta_key'] = 'nickname';
$this->query_vars['meta_value'] = $_REQUEST['s'];
$this->query_vars['meta_compare'] = 'LIKE';

Any help?
Thank you.

One thing I hate is bumping into a rare problem on the internet that someone else has bumped into and the thread has died with no answer.
SO
After several hours of trial and error and investigation, I finally solved my problem.

This is how the constructor looks like in side-user.php:

function __construct( $query_vars ) {
		$this->query_vars = $query_vars;
        $this->query_vars['orderby'] = 'display_name';
        $this->query_vars['order'] = 'ASC';
        $this->query_vars['meta_query'] = array(
            array(
                'key'     => 'nickname',
                'value'   => $_REQUEST['s'],
                'compare' => 'LIKE'
            ));
}

This is how the function abstract_query looks like in directed-connection-type:

private function abstract_query( $qv, $which, $output = 'abstract' ) {
		$side = $this->get( $which, 'side' );

		$qv = $this->get_final_qv( $qv, $which );

        unset($qv['search']);

        $query = $side->do_query( $qv );

		if ( 'raw' == $output )
			return $query;

		return $side->get_list( $query );
	}

The key here is unset($qv['search']);.
The meta_query was absolutely correct and functional but it turns out it was intersecting with the original code.
Example: when typing ‘a’ in the input field, the results had to have ‘a’ in user_login AND in the nickname.

I hope this helps someone who eventually runs into something like this.