You could probably bungee, or use cam straps to tie the factory case to the top of the rack. With a little practice, it would attach and remove pretty quickly.

I ride with a backpack quite a bit, and I'm sure you could find one that would fit. Some have a special computer pocket, although I'm not sure the size.

The old 17"+ computers were behemoths, but I would assume new ones are getting lighter. Things like a CD/DVD drive would be nice, but really I don't use it that frequently (other than when I was doing IT support).