In an ideal world I'd love to be able to 'nest' meta_query arguments in WP_Query, but I realize the limitation in the structure of this gist being too loose to implement programmatically, but I'd like to think about ways to possibly accomplish something like this in a more efficient way than filtering posts_join

gistfile1.php

<?php

/**
 * My events have two Custom Fields, one for a start date and one for an end
 * date. Start dates are required, but end dates are optional. That's because
 * some events are one day only, while others might span multiple dates 
 * (e.g. conferences). Dates are stored in MySQL format.
 *
 * I want to pull events that meet these criteria:
 *     - Event has only a start date, and it's after NOW()
 *     - Event has a date range with an end date after NOW()
 *
 * That way, if a conference is currently going on (started previously but ends
 * in the future) the listing doesn't go away.
 */

$args = array(
    'nopaging'          => true,
    'post_type'         => 'my_event',
    'meta_query'        => array(

            // we want any of these 'conditions' to be met
            'relation'      => 'OR',

            // 'condition' 1: has only a start date, and it's valid    
            array(

                'relation'        => 'AND',
                
                // start date isn't empty
                array(
                        'key'       => 'event_start_date',
                        'compare'   => 'EXISTS',
                    ),

                // start date is after NOW()
                array(
                        'key'       => 'event_start_date',
                        'value'     => current_time( 'mysql' ),
                        'compare'   => '>=',
                        'type'      => 'DATE',
                    ),
            ),

            // 'condition' 2
            array(

                'relation'        => 'AND',
                
                // start date isn't empty
                array(
                        'key'       => 'event_start_date',
                        'compare'   => 'EXISTS',
                    ),

                // end date isn't empty
                array(
                        'key'       => 'event_end_date',
                        'compare'   => 'EXISTS',
                    ),

                // end date is after NOW()
                array(
                        'key'       => 'event_end_date',
                        'value'     => current_time( 'mysql' ),
                        'compare'   => '>=',
                        'type'      => 'DATE',
                    ),
            ),

        ),
    );

$events = new WP_Query( $args );

Just added this to Pods 2.3 (nested relations, as many levels deep as you need):

https://github.com/pods-framework/pods/blob/2.x/classes/PodsData.php#L2012