One way you could do it is this, as a global announcement is that, global.
Custom Headers and footers.
Each forum can have unique Headers and footers.
This is like reversing what you have now.
In your sponsors forum update custom Headers and footers to what you have for html includes and forum footer. This will display the origional header and footers you may have in place.
So create a banner or footer.
Under the Main forum settings of html includes and forum footer.
Add the code to the custom header footer respectively.
Include in the code a link to the Topic you may have created, for example if you click on the banner it takes you to that topic.
So you should now have your new banner everywhere except your sponsored forum as you have used unique header footer options.
It sounds long winded, but I have over 50 individual forums so thats how I would work the problem.
There may be other ways to do this.
