[nyanpasu64]

You can override EDID in the kernel options (https://foosel.net/til/how-to-override-the-edid-data-of-a-mo...), but I don't know if you want to add a virtual monitor (unsure if https://askubuntu.com/questions/453109/add-fake-display-when... works).

[mlukaszek]

Have a look at https://github.com/DisplayLink/evdi and (shameless plug, toy project, be gentle) https://github.com/mlukaszek/evdipp