Add virtual X11 screen to use via VNC

I'm trying to add a second screen to my ubuntu 14.04 gnome setup, via a VNC client. To be clear: I don't want to connect to a different session, but the same. However, I want to see a different screen, not mirror the existing physical screen. I just want to extend the existing display with a "virtual" second screen that I can reach through VNC, instead of a physical one.

Much to my dismay, I haven't found a way to achieve this, let alone a simple one. Is there a way at all?

The VNC client will be an iPad app in this case, but it could be anything with a VNC client really, such as an old laptop that I wanted to use as a second monitor.

This might be a duplicate of "Adding a virtual screen to xorg.conf 11.04 (third display over vnc)". However, there is no answer to the actual question, and an answer suggesting a different route is accepted, so I'm posting this one to try and find an actual solution.

You can use virtscreen on linux to use an ipad or other computer as a secondary screen.

sudo apt-get install x11vnc
wget https://github.com/kbumsik/VirtScreen/releases/download/0.3.1/virtscreen.deb
sudo dpkg -i virtscreen.deb
virtscreen

The app is made by Bumsik Kim who commented above (but it's been a year and they've not posted their solution, so I did).