This is similar to what is already available for SVN and Git. Newer versions of Perforce (2008.1 and up) can put the url directly in the counter value, while older versions require the slightly hackier method of embedding the url inside the counter name.

Tested both methods against a 2008.1 Perforce server and the counter name method on 2007.2 and was able to successfully post reviews in all cases without specifying the Review Board url anywhere else.