Have you already tried such a thing or investigated on whether it would be possible or not? What is your opinion on that? Please advise.
When it says that it's not compatible with Windows RT platforms, that means you can't install the voice/video add-on on mobile platforms. That includes Windows RT, Android, etc. This is common to add-ons in general. There are various extensions available for Chrome that I can't install on Chrome on my Android phone, for example. So any app you build for Windows 8 won't work on Windows mobile, since you can't install the required add-on.
If you just want to build a Jabber voice/video app for Internet Explorer, then it should work fine on Windows 8. If you're using PCs, then the limitation for Windows RT doesn't matter. It should work fine. Whether or not you can turn it into a Windows 8 app (something you can put on the Windows 8 app store) is something I can't answer, since I don't know what Microsoft requires for that kind of thing. I believe Microsoft wants all apps to run on all platforms, so I'm guessing it wouldn't be appropriate for a Windows app store.