Kathy, here's a sneaky way to do it with no hack needed.
Write a conditional. If forum=x and postcount=1. Wrap the conditional around a second copy of your postbit with the username and message hard-coded in the template. Put this whole thing at the bottom of your postbit template.
Then, when that first post is made in that forum, it will LOOK like a second post is immediately made in reply. That reply could have all the necessary info.
Amy