I recently learned about evdi, an out of tree Kernel module that can add as many virtual outputs as you want. Ideal for things like this, or where you'd normally use a dummy plug.

The link for anyone interested: https://github.com/DisplayLink/evdi

It's unfortunate it wasn't accepted in-tree because while it was initially developed for a specific commercial product, I do think it has the potential to be generically useful.

The ability to conjure up a display "in software" & then just have the rest of the system automatically treat it as any other physical display is pretty powerful.

There's a couple of projects on my long "projects to do" that would make use of it...