I have been working on similar functionality this week and I may have a solution for you. It consists of some changes to the template files: tpl_form.php and tpl_index.php
It appears to work but hasn't been fully tested - the main area I am unsure of is whether the mod will work with all the error and information messages that may be returned on submitting the comment form.
You can see the mod in operation at http://www.foxbottle.com/joomla
- hit the JComments link. You can enter any comments you like here - this is a test site I have set up for Joomla so I don't mind
If you would like the mod let me know and I'll forward the files to you.