I'm rendering with Cycles in the viewport (not with F12) and would like to get that viewport output as an input node into the Compositor.
How can this be done?
Well there is a way but it really is a cart before the horse thing. If you really need the image you can only get it by a screenshot, which will be limited to an 8 bit depth and the size of your viewport, obviously.
When it is saved you can then use a file node to import it into the node editor. As I said this is a nuts way to do it but it can be done.
Don't do it though use F12.
As far as I know you can't, the viewport rendering is just used as a preview as you develop shaders and lighting.
The way you get the output is from a full (F12) render, then in the compositor:
You can't, because the viewport render is only a preview. In fact, you shouldn't rely on the viewport for determining the final output, actually, because it doesn't represent an accurate view of what the final render will be and wasn't meant for that purpose.