Virtual video capture device? Is it possible to create a virtual video capture device in Mac OS? 

Sort of like how Loopback creates virtual audio devices...?

I'd like to have a setup where I can take the input from say my laptop's HD camera, processed it with opencv, and then pipe that to some other program that is pretending to be a video capture device (ie it would present itself on video input selection dialogs).

Is it possible to create such a virtual video device, similarly to how audio ones are made?
You can create such a virtual camera device by using the Webcamoid software:

https://webcamoid.github.io/

It supports the virtual camera feature in version 8.5.0 and later.