Is there some way to provide the user a list of existing content in a CPT

I’ve run across this before. What I did was use get_posts() to return a list of the pages in the site and making the pages options in a drop down menu in a meta box.

// $your_args is an array that you've previously declared to get all the pages.
$pages = get_posts($your_args);

foreach( $pages as $page ) :
?>
<option value="<?php echo $page->ID; ?>"><?php echo $page->post_name; ?></option>
<?php
endforeach;

When you save the post, it will save the page’s ID in the custom post’s post meta.