I finally got some first stencil shadows to work. They don´t yet work with more complex objects due to the kind of normals the shadows need, but that is going to be easy to fix. A bigger problem is, that my silouette detection is quite slow at the moment. There is space to optimize it, but I am not sure if that is going to be enough.
However, here is a first screenshot:


Fixed the normal "problem":


and some house I´ve thrown together in blender (shows pretty well, that things are slow at the moment^):