Howard,
because the CSS has a script language, you can basically do whatever you want.
You could have a script running on the server to get the server cpu and save it into a file.
Via the CSS script language, you could retrieve this value and decided to use or not the server.
There is also something called DFP - dynamic feedback protocol. If your server vendor support this protocol you can use it to report server status to the CSS.
Regards,
Gilles.