Foren / Cotonti / Extensions / Support / jQuery modals in plugins

You could try of removing this: 

cot_rc_add_embed('modalexample','
    $(function() {
    $( "#dialog-modal" ).dialog({
      height: 140,
      modal: true
    });
  });
  ');

And just including it directly in the plugin. 

Also $plugin_body isn't needed anymore, I suggest looking at more recent plugins and seeing how they are coded.