If you want to clear local session status variables this is easy, simply execute FLUSH STATUS. Unfortunately doing the same for global status variables is not as easy.
That is until now. In the Drizzle7 beta release 2010-11-08 is a new command: FLUSH GLOBAL STATUS
I created this command to fulfil a Rackspace feature request but it appears this has also been a MySQL feature request in the past. The implementation would be very different in MySQL since the status variables and sessions work differently, but it would certainly be good to see in a future release, and it appears patches already exist for it.