Example of how you can filter registrations based on promotions.

tw_ee_filter_registrations_based_on_promo.php

<?php // Do not include the opening PHP tag if you already have one.

add_filter(
    'FHEE__Registrations_Admin_Page___get_where_conditions_for_registrations_query',
    'tw_ee_filter_registrations_based_on_promo',
    10,
    2
);
function tw_ee_filter_registrations_based_on_promo( $where, $request_params )
{
    //Check we have a Promotion ID set on the request.
    if( !empty( $request_params['PRO_ID'] ) ) {
        //Add where conditions to limit registrations based on the Promotion ID passed.
        $where['Transaction.Line_Item.OBJ_type'] = 'Promotion';
        $where['Transaction.Line_Item.OBJ_ID'] = (int) $request_params['PRO_ID'];
    }
    return $where;
}