I did a PCB design for a small extractor board and ordered. Also the components have arrived already - I just have to wait on the boards to arrive to actually start trying cable configurations out.

Having played around in Simetrix with transmission lines and stubs, I'm now convinced that it is crucial to keep the stub lines to the extractor very, very short. I am not a great PCB designer and all I managed is to keep the DVI tracks to the chip under a centimeter (but probably they have wrong characteristic impedances). This will be hit or miss without any way to measure the signals directly... Fingers crossed!

If it doesn't work I can always still opt for a 50/50 power splitter solution that limits the main cable's length, or a complete DVI repeater (expensive).

When the boards do arrive and have been tested, I will publish the PCB design here also.