Timed Comment Enabler/Disabler

I'd like to be able enable/disable/enable the ability
to leave comments.

My specific use case is that Im running an auto
webinar where Im going ask attendee's to say
hello and in the comment area during the 1st
10 minutes of the presentation.

Then close commenting (but want to continue
to show the comments previously left)... then
reopen comments for questions.

Any recommendations on how to do this?

PS: Is there a way to place a "comment box
shortcode" in the body of the WP post and
would override the the checkbox "allow