Hi Greg,
What you may do is manually show some div on top of the iframe, giving the message "your message is being sent, please wait", when user presses the submit button (you will need to add a simple javascript to validate_form function to switch your text from hidden to visible).
Also, what we are planning to implement soon, is to actually give the message "Your message is being sent, please wait. Thank you!" on our side, so users will see this message instead of white screen, when page is submitted. Unfortunately I can't give you an exact date for this implementation, but I am sure this will be done within two weeks period.
Thank you
Victor