I don't think there is an easy way to do this. Web Report Studio just does not support "count distinct measures". I was at a SAS conference where they demo'ed 4.2 (?) that's included with the SAS 9.2 suite and that didn't appear to have any substantial functionality either. Just a bunch of interface improvements.
At my company we used a 3rd party product Futrix that sits on top of SAS and basically is a much more advanced version of Web report studio. It has count distinct measures built in. Basically, in terms of functionality Futrix is light years ahead of SAS Web Report Studio. Of course, it is not a free software, so I don't know how much this will help.
Bottom-line: Don't look to SAS Web Report studio if you want to implement any kind of serious web-based reporting.