Hey @mawa!
Thanks and thanks! 
Yes, the reflections are far from real and I have just noticed, that the spheres are reflected upside down and the scaling ehm, it is another story, but I leave the remaining issues to be fixed by someone else, maybe even by you
Also it would be cool to calculate the relative position of reflected objects from the reflector origin and decide dynamically, which objects can be reflected by a given reflector.
Originally I wanted to share only the concept of the transparent custom shader and didn’t pay too much attention to the reflections, because as I already wrote, I don’t need them in my project, now. But I felt a bit dumb after demonstrating the shader with the cube reflection texture 
so I tried to fix it by the new PG.
To tell the truth, I don’t have too much time to play with these things now and the lack of knowledge of some concepts also bothers me, but I will get back to this reflections stuff sooner or later. As you have noticed, I am a newbie here, so please, if you or anyone else have any suggestions I will gladly implement them, or will try to, at least.
Thanks!
EDIT: I started a new thread on the new PG here: Building windows with inside transparency and "real" outside reflections