See, but that's just half way through. The only thing that changed so far is using a DRM-compatible module for the built-in GPU (vc4).
Now, the trick is to add `udl` (for older devices) or evdi (for newer) on top of that, and call `xrandr --setprovideroutputsource ...`.
So - what you could do now is to try to build evdi (I remember I used rpi-update and rpi-source combo for getting kernel sources and building the module afterwards). Then, despite lacking DLM, you can try to build and run monitorsim from
here.
Alternatively, if you have an older device that can be driven with UDL, try this instead.
If everything works, meaning that you still get to see the main HDMI output, plus additionally you see the second screen (in a window, in case of monitorsim), please do report back.
Note: I'd recommend to experiment via ssh.
Fingers crossed!
Cheers,
Michal