It would be awesome if it were possible to create a virtual output whilst programming.
I'm currently programming a show on an Amba and will be feeding 4 x 1920 screens via an FX4 when I get on site. However whilst I'm programming I don't have the physical screens, so it would be great to create a virtual 4K output. Currently my workaround is to scale down my output to one 1920 monitor.
It seems you can set a viewport as a source in the same dropdown as the mixes, but that only works when you have a viewport patched to an output. So I guess that loops back around to your first request of being able to patch to a virtual output. Or at the very least being able to patch a mix to a viewport without patching that viewport to an output.