Oh, it does allow you to add buttons to panels. It doesn't allow you to add area types, dialogs and tabs in the properties area. Also there is no such thing as

Oh, it does allow you to add buttons to panels. It doesn't allow you to add area types, dialogs and tabs in the properties area. Also there is no such thing as a button callback in Blender UI. The only thing you can do is insert a button which calls an "operator". You can define your own operators of course, but this only works as long as you don't want to generate the buttons dynamically. Then you'd have to generate operators dynamically, and things only go downhill from there...