I'm afraid it's not possible, but you could use a custom HTML form element instead to hard code some HTML to be output. Here's an example:
$qa_content['form']['fields']['plugins']=array(
'label' => qa_lang_html('admin/plugin_pages_explanation'),
'type' => 'custom',
'html' => '<UL STYLE="margin-bottom:0;">'.$listhtml.'</UL>',
);