I'm not allowed to drill into the walls to pass a cable through, so I see no other option than passing a flat conductor between a window and its frame. Products for doing that exist, but I am concerned that they might not be durable enough in the long term. Here is an example, a flat copper braid crimped into connectors sold by wallbox24.de:

If these don't hold up they would probably not only become a fire hazard, but also be too expensive to replace to make the whole endeavour financially viable. I doubt that they are certified by anybody. Maybe I should ask the dealer.

If these should not do the job reliably, I had some other approaches to explore:

• a flat (maybe 0.5 mm) but wide copper sheet with isolation around it that can be bent around the window frame. It could be terminated with large WAGO clamps to connect to regular solar power leads.
• Maybe certain types of flex PCBs could be robust enough. It could be possible to add traces to detect failures in the power traces.

I think this problem is the bottleneck for the project. If there is no safe way to do this, the whole idea is worth nothing. If you have any thoughts on this I'd be happy to hear your suggestions.