Haven’t heard anything back, but I have found a one-line workaround that seems to work just fine. There’s no way to hook the function so the adjustment must be made in the plugin itself.
Around line 189 in the file pg-context-sidebar.php find the text
<td><textarea id="pgeek-cs-content" name='pgeek_cs_content' rows=8 cols=90 ><?php echo esc_attr($content * ); ? ></textarea>
And replace it with
<td><?php $my_settings = array('textarea_name' => 'pgeek_cs_content'); wp_editor( $content, 'pgeek_cs_content', $my_settings ); ?>
Of course if the plugin is updated you have to reapply the fix. (Unless the fix were in the update itself.)