For CX flights, at least out of North America, you only need to worry about "H" class availability. All other subclsses don't really work for ex-US flights.

Yes if you want to check specific classes, this tool will come in handy. But it's also just as easy to call CX res and ask them to do a scan search for the next say, 10 days to ask for availability in "A" space for the RTWs.

One thing I guess CX's website is better is that you can check for award space availability if you're a Asiamiles or Marco Polo member.