Can't Get Custom Field to Display Outside Loop

Please help. I’ve been at this for hours trying to figure out how to make this work.

I’m using a Page template to build a custom RSS Feed that includes my URL shortener prefix. I need to include the shortener as a custom field value, but I can’t get the custom field value to display.

Here’s the code I have so far (with the working fields stripped out for brevity). Please tell me how to fix this so that it works.

<?php $query = new WP_Query( 'showposts=-1&cat=-3,-8' ); ?>
<?php global $post; ?>
<?php if ( $query->have_posts() ) : while ( $query->have_posts() ) : $query->the_post(); ?>
<item>

<link><?php echo get_post_meta($post->ID, 'permalink_prefix', true) ?><?php the_permalink_rss() ?> </link>

<?php do_action('rss2_item'); ?>
</item>
<?php endwhile; wp_reset_postdata(); ?>
<?php endif; ?>