It’s odd, but this still doesn’t work –
$args = array( 'post_type' => 'project', 'meta_query' => array('key' => 'homepage_slide', 'value' => '1'));
I will check out a debug plugin and see if I see anything wrong.
Just curious, if you were trying to do the same thing (add slides to a slideshow only if the homepage_slide field was completed), how would you write the query?