I create now a script that do that:
http://files.mundoopensource.com.br/of_serverinfo_prtg.py
You can put it in your cron to create the file.
./of_serverinfo_prtg.py online users
<html>
<body>
online users [4]
</body>
</html>
Is it sufficient?
Regards,
Marcelo Terres