Okay, I updated the diff so that it works with trunk svn.

I took the (ugly, static-typed) Java and kinda converted it into prettier pseudo-Python (my Python is sadly kinda rusty, so it might not be perfect):

# The second argument is a list of all the repositories on the ReviewBoard 
# server.
def findRepository( project, repositories ):
    vcs = project.vcs
    localInfo = vcs.getInfo( project.baseDir )
    localUuid = localInfo.repositoryUUID
    # This gives you the directory of the repository you've checked out.  So if
    # the repository is "http://svn/repos" and you checked out
    # "http://svn/repos/trunk/reviewboard", this will return "trunk/reviewboard"
    localRelative = getRelativePath( localInfo.URL, 
                                     localInfo.repositoryRootURL )

    client = ReviewBoardPlugin.getClient( project )

    svnRepositories = filterSvnRepositories( repositories )

    for repository in svnRepositories:
        try:
            info = client.getRepositoryInfo( repository.id )
            remoteUuid = info.repositoryUUID
            if localUuid != remoteUuid:
                continue;

            remoteRelative = getRelativePath( info.URL, info.repositoryRootURL )

            # Now, if the remote relative path is "trunk" and we have
            # "trunk/reviewboard", this will return "reviewboard".  This is the
            # path we need to hand off to reviewboard as the base diff path.
            relative = getRelativePath( localRelative, remoteRelative )

            # This is the same repository, but they're different paths.  The
            # local checkout might be "trunk/reviewboard" and the one on the
            # server might be "branches/2.0".
            if not relative:
                continue;

            return ( repository, relative );
        except Exception:
            pass

    return None;