Thank you for your response!
I actually played a little with the Details but it turned out to be too complicated for my poor Architect's brain! Even my 1-year Engineering education didn't help either!
Scripting is just not my way of making things happen!
So I went for an classic Window Ribbon and an extra layer for a wall for the transom. This way I have my Oversize for the Technical Drawings and the transom for the 3D visualization. Viewed from a distance no one will see which part of the window is openable and which not!