so, let's say I come up with a list of files that you want to replace with Mac versions, the next question would be "Where are you going to get those files?" If you have access to a Mac with those files, you can copy over the entire WoW folder. Problem solved. Much easier than you mention.
But let's try answering it anyway:
So, .exe and .dll files will have to be replaced. Those are going to be written exclusively for the Windows Environment (Win32 api calls...), so you can't save them. MPQ files are Blizzard data files, and those can be saved. Looking at the other files I have in my WoW install folder there's a bunch of other files you don't need, like patch and downloader files...
There's a few other extensions that are just data files:
I also have a manifest file. That's microsoft specific, but it's also a data file. It should be associated with .dll files, so it doesn't need to be copied.
According to the blue poster on this thread:
There's a bunch of folders you can just delete, because they'll get built automatically by the system. Those are the files that you can keep and copy over if you want, because they're just data files, but they also aren't strictly necessary to have WoW work. What is it you're optimizing for?