If you're talking bout covering the phone, then the real question should be why you would need a case when you'll still put the mod on top of it? Just wondering what use it will serve if you'll put the mod on top still
If you're talking about cases in general, then not to worry there'll always be case makers that make one for the Z. (Though I doubt this is what you're asking about)
Now, if your fear is that the mods might scratch the body of the phone, then never mind, because it won't. They're not fools, if the style mod doesn't scratch it, then the other mods wouldn't.
Or are you talking bout a case for the Mods? Lol