SCRIPT_NAMEを空にすればいいじゃなーい。

try:
    from trac.web import fcgi_frontend
    def rewriting_dispatch_request(environ, start_response):
        environ['SCRIPT_NAME'] = ""
        return fcgi_frontend.dispatch_request(environ, start_response)
    fcgi_frontend._fcgi.WSGIServer(rewriting_dispatch_request).run()
except SystemExit:
    raise