I added functions in my Draw2Paint addon to be able to pass an image directly to the Compositor and bring it back to the same node tree in the Shader Editor. The manual switch is the 2Compositor/2Shader button.
With the image node selected, the press of Image2Compositor sends the image to a new image node connected to the viewer node and turns on the backdrop if not already active.
Then you add what you want in the chain for editing the copy image in the Compositor, and then press Compositor2Image to send the result to a new Image Node in the Shader editor. You can then connect that to use it where you want. To save it, you will need to pack it in file or save to a directory location using the Image Editor.
My thread here on BA is here.