Solved it. For anyone else wondering, this is how I did it:
<?php
$args = array(
'numberposts' => -1,
'post_type' => 'portfolio',
'showposts' => 1,
'orderby' => 'rand',
'meta_query' => array(
'relation' => 'AND',
array(
'key' => 'client_quote',
'value' => '',
'compare' => '!='
)
)
);
$client_quote = new WP_Query( $args );
?>
That randomly picks one post from the Portfolio custom post type, as long as the client_quote custom field contains some text.
