WSA + Supporter

I'm aware that the WSA tags are added to the big100 themes pack, but I want a way to only show the WSA ads on "Free" blogs, and not on the supporter blogs.

I tried adding the <!--wsa:context-name--> to the Before/After ad spots in the Supporter settings page, but it just outputs it as text, and doesn't send the text through the WSA engine.

Am I supposed to make some "display/don't display" trigger that checks whether the blog is a 'supporter' blog or not?