I am trying to get the File Output Node of the compositor to create a new file with a different name (appending a number would be best) if one already exists. Controlling the name with some extra input perhaps would also work.
I'm in this situation because the time is being controlled by a speed track on the VSE. The frame are actually sub-frames, but the File Output Node generating the output file name based on the integer frame number. The result is that there are less files than there should be, because multiple sub-frames end up overwriting themselves.