Display one post in different formats conditionally

A potential solution would be to first get rid of the Post Template plugin and just use the default single.php to render single post views.

Then add a get variable onto the end of your imagemap links, so they’re something like domain.com/post-name/?view=lightbox

Then filter the single_template to load your lightbox view template when the GET var is detected:

function wpa_lightbox_single( $template ){
    if( isset( $_GET['view'] ) )
        $template = locate_template( 'lightbox-view.php' );

    return $template;
}
add_filter( 'single_template', 'wpa_lightbox_single' );