Thanks to comments on Jenny's article, another idea came into my mind - VGA to HDMI adapter. I have digital RGB/Bright/VSYNC/HSYNC signals out from my "ULA" rp2350b and only need to translate them to TMDS. There are some specialized chips like tfp410 for this purpose but probably much easier would be using another Raspberry mcu  and do it in software. Looks like something like rp2040-Zero must be good for this task:

The task is to catch VGA output from "ULA" GPIOs:

and translate them into TMDS for HDMI output. Looks like it should be possible to put the Zero board "instead" of resistors R49 - R51 on existing PCB:

The resistors still have to be installed somewhere though.

To be continued...