yup, I have 10 wow directories, all of them symlinked, and I just had to update the client once today morning and resymlink new files (just two new files in the main directory, battle.net.dll and msvcr80.dll) and all my clients were updated. took under 10 minutes.

But yes you can copy over the updates folder, and when wow launches it will download the patcher (you can copy that too) and it'll auto update. Kind of a long time if you are doing 4-5 wow installs though, should seriously consider learning how to junction/symlink.