Solved!

It seems that the plugin I was using to create my custom field was the culprit. Once I realized that WP had custom field functionality already built in, I found that adding the following to display_post_tab_content was all I needed to display a WP-generated custom field in tab post content:

$output .= 
'<span class="myClass">'. get_post_meta($post->ID, 'myCustomField', true) . '</span> ';

(My solution did not work for the custom field created via the plugin which shall remain nameless. Disabled and deleted!)

Thanks again for Popular Widget!