Drew Gourley

Further Investigation reveals this to be a more complete solution:

function dg_override_404() {
	global $wp_query;
	$args = array( 'public' => true, '_builtin' => false );
	$post_types = get_post_types($args);
	if ( in_array($wp_query->query_vars[post_type], $post_types) && $wp_query->is_404 == true ) {
		$wp_query->is_404 = false;
		$wp_query->is_archive = true;
		$wp_query->is_post_type_archive = true;
	}
}
add_action('pre_get_posts', 'dg_override_404');
function dg_override_template() {
	global $wp_query;
	$args = array( 'public' => true, '_builtin' => false );
	$post_types = get_post_types($args);
	if ( in_array($wp_query->query_vars[post_type], $post_types) && $wp_query->is_404 == true ) {
		include(TEMPLATEPATH.'/archive.php');
		exit;
	}
}
add_action('template_redirect', 'dg_override_template');

I still need to find a way to change the headers to 200 instead of 404 and to circumvent the ‘Page not found’ from showing up in the title of the page.

Hey guys, ran into this myself with some hard work created this solution:

You can toss this into functions.php in your template and it will give you the desired effect.

function dg_override_404() {
	global $wp_query;
	$args = array( 'public' => true, '_builtin' => false );
	$post_types = get_post_types($args);
	if ( in_array($wp_query->query_vars[post_type], $post_types) && $wp_query->is_404 == true ) {
		$wp_query->is_404 = false;
	}
}
add_action('pre_get_posts', 'dg_override_404');
function dg_override_template() {
	global $wp_query;
	$args = array( 'public' => true, '_builtin' => false );
	$post_types = get_post_types($args);
	if ( in_array($wp_query->query_vars[post_type], $post_types) && $wp_query->is_404 == true ) {
		require TEMPLATEPATH.'/archive.php';
		exit;
	}
}
add_action('wp', 'dg_override_template');

Basically what this does is only firing when your on an non-built-in post type that is returning a 404 error, which is only the case on empty post type archive pages. It switches the is_404 argument to true and forces it to use the archive template. I’ll investigate this more tomorrow, because it probably requires some refinement.

Ha, nice.

Really odd is that it works for me both ways…

Sorry to hear that this didn’t work, but there is one difference between yours and mine. I changed the way the condition was formatted. This is a long shot, because everything you’ve done SHOULD make it work, but it hasn’t… Here is the exact code from my page.php:

<?php if (class_exists('MultiPostThumbnails') && MultiPostThumbnails::has_post_thumbnail('page', 'secondary-image')) { ?>
<div id="secondaryImage">
	<?php MultiPostThumbnails::the_post_thumbnail('page', 'secondary-image'); ?>
</div>
<?php } ?>

And the div isn’t necessary, its for CSS so:

<?php if (class_exists('MultiPostThumbnails') && MultiPostThumbnails::has_post_thumbnail('page', 'secondary-image')) {
  MultiPostThumbnails::the_post_thumbnail('page', 'secondary-image');
} ?>

make sure that the post type matches up with the post type you declared when you register the post thumbnails.

The author shows

$thumb = new MultiPostThumbnails(array(
    'label' => 'Secondary Image',
    'id' => 'secondary-image',
    'post_type' => 'page'
    )
);

but then incorrectly tells you to put this in the template:

<?php if (class_exists('MultiPostThumbnails')
    && MultiPostThumbnails::has_post_thumbnail('post', 'secondary-image')) :
        MultiPostThumbnails::the_post_thumbnail('post', 'secondary-image'); endif; ?>

if you registered it as post-type => page, then you’d have to use:

<?php if (class_exists('MultiPostThumbnails')
    && MultiPostThumbnails::has_post_thumbnail('page', 'secondary-image')) :
        MultiPostThumbnails::the_post_thumbnail('page', 'secondary-image'); endif; ?>

This got it all working for me.

Don’t forget, this is really versatile! You could literally register ten or more post-thumbnails by giving them unique identifiers when registering them. Exciting stuff!