As usual, just a quick idea/suggestion for your development backlog lest I forget. :-)
To my mind, it would be incredibly useful to bridge the workflow boundaries, either via copy&pasting (as I previously suggested) or, more structuredly, via "entrance" and "exit" nodes for data.
This would open up a whole new world of possibilities - and complexities, as I'm well aware. However, the necessary meta-layer could be useful for metadata management as well, e.g. for finding nodes across flows (which would require and/or enable flagging and tagging).