single comment only feature

Hi,
I need to implement a feedback system in my site. By this i mean allowing only one comment/feedback for a post from a specific person. The criteria for identifying the person would need to be validated on the commenters email address.

Ideally if feasable, i would really like a system whereby the writer of the post and commenter devise a secret password only known to them both. The commenter then has to enter this password successfully to leave the comment.

Is there a plugin that comes close to this functionality that i can modify or will i need to write it myself which i am prepared to try.

BTW – I know there is a password feature built in to wordpress that protects post – but i wish to allow everyone to read the post but only one person to comment upon it.

With thanks in advance —