I think I'd have two scripts: save_form.cgi and load_message.cgi. You submit your form and it goes to save_form.cgi, which saves the data. save_form.cgi returns a simple page that just says "message saved" or whatever, and has a bit of javascript that changes the right_frame's location to something like "load_message.cgi?id=1234", where 1234 is the recently submitted message's identifier. Then just have load_message.cgi read the thing they just posted, and display it.

I should warn you though, I'm a little sleep deprived from being sick lately, so there's probably easier/better ways to do it. That was just my initial reaction to it