Update: I found a workaround! (I am very, very happy right now)
The trick was to delete the GL driver kext for the Intel GPU ( /System/Library/Extensions/AppleIntelHD3000GLDriver.kext).
Now I can use the monitor attached to the iGPU, albeit with some small artefacts, and the nVidia OpenGL renderer is the only one left so it is the one which is used. Even displaying a composition window on the monitor attached to the iGPU works and looks fine.
(P.S. of course I haven't deleted the kext, I moved it to another folder so I can restore it later If I need to.)