If you need to physically accumulate the drawings, I would see two options:
1) memorize drawings in a list of draw commands, so you can re-draw everything when the node needs to be updated
2) Use a RenderTexture to accumulate the drawings. However I have no idea how it would work in the 2D engine, I suppose you can have a secondary Viewport with a RenderTexture as target, disable frame clearing, then you would use that texture in the main viewport to display it. (and of course, as I am trying that right now, it lamentably fails... EDIT I found it, getting there. I'll maybe show an example if I get it to work :D)