My first idea was to use a telescopic rod that is clamped into the sides of the window for mounting the hardware. This principle is also used for rods that are clamped into door frames for doing push ups. Those hold 100 kg easily, so I think that this is a promising approach to explore.
For my experiments I bought a telescopic rod for mounting cat nets on balconies. It is sturdy but overpriced. If my experiments are successful, I will publish details on building one from scratch.

I constructed a simple frame for holding a 100 W solar panel at an ideal angle of 32° from U-shaped aluminium profile and 3D-printed parts. The prototype is mounted in the window of friend's houseway higher than planned just for easier access when tinkering.

Takeaways:

• Everything seems quite sturdy. I'd prefer bigger pads on the telescopic rod though.
• I felt more comfortable when the outriggers could freely rotate on the rod rather than when they were clamped down, since vibrations might loosen the rod over time.
• The threaded rod strut flexes a bit, I would use two when I decide to explore this approach further.
• I'd replace the printed parts on a U-profile with square profile and U-clamps.