There might also need to be some UI for people to add properties the names of which we don't know at all. IOW, for those cases where the properties of a node aren't restricted. In fact, most of our resources fall into that category. I can add a property of my own conception, in a defined namespace. In short, we don't always know the names of properties people might want to add.

Maybe a "Other..." selection that offers a box to select a namespace and fill in a name? Or maybe we key it off of a separated namespace selector with which users must begin their edit?