Lee, that is the fear of all measurers, and there is no good answer. Personally, I would not post negative comments regarding an event, as that invites bad karma.
But, if I were in your shoes, I would send the RD an email, and let him know that, due to his lack of payment, you will be left with a few choices, which include requesting that USATF rescind his course certificates. Let him know that if that happens, anyone looking for the certificate numbers will find that the certificates are "expired".
Now, between you and I, I don't want to use the course listings for payment enforcement. Thus the phrasing "request that the USATF rescind...". "Request" does not mean that it will happen. Give him a two-week deadline to make payment.
In that email, you can remind him that the Web reaches many people, and that one of your options is to let people know that his event has not paid the bill. Tell him he has time to prevent all adverse action, if he just pays the bill as agreed upon.
Finally, if it is enough $$, you could also let him know you have the option of filing a claim in Small Claims Court, which should result in him paying any legal fees you incur.
Spell it all out, and let him make the decision as to what happens - he either pays, or you pursue your aforementioned options.
Good luck!