This project is ongoing for a while. First attempt was to wire up a cheap 4 input 2 output HDMI matrix to an esp32. This was abandoned after it was kind of unstable.
The current approach is to use a: Ligawo 3080026 4x1 HDMI switch acquired from Amazon which has a serial interface.
We reimplemented the serial protocol, as the windows software is a) well a windows software, b) a shitty one.
Now we have some nice bridge between MQTT and the HDMI-switch.