It's not really worth it in my opinion. From my experience you have to let each set of updates slipstream into each single version and THEN end up with a big image. I did this a couple years back and just made me realize it wasn't worth repeating over and over every so often with a new set of updates every month. I would say you're better off just keeping an updated set of .msu's or whatever updates and then integrate into whatever version you need at the moment.
btw, when I did do it it wasn't very hard. You just use Windows Toolkit and select ALL versions to integrate them into if I'm not mistaken (which I might be since it's been a while).