Are cloud providers really an issue here? If you as a service provider want IPv6 and your hosting provider doesn't support it, you can relatively easily switch. But if your users don't have IPv6 (and many don't), you're basically screwed - you can't change your users' ISPs and they certainly won't either.

Also, what would be the security implications of this? The best I can come up with is networks that treat these addresses as internal and services inside trust them by default, but such networks wouldn't allow this traffic from the WAN side anyway, right?