template_redirect for single posts w/ custom fields

Question

The problem with your code is that you’re checking is_single() when your plugin is first loaded, before the global query has run, so is_single() still returns false. You should move the is_single() check within your my_template function:

function my_template() {
    if(is_single() && have_posts() && 'Custom' == get_post_meta(get_the_ID(), 'Design')) {
    include(PLUGINDIR . '/supersqueeze/all.php');
    exit;
    }
}
add_action('template_redirect', 'my_template');