I wonder why that couldn't be solved in the following way, splitting the problem in several task.
Calculate FPS of the original file. This can be achieved with parabox "frames per seconds" node: play the movie for a few seconds, hold the value this node gives.
Process the movie with "step movie frames" technique I showed in the other post, and put every processed frame in a list; no worry if frame elaboration needs huge time;
With a fire periodically node, triggered every [original FPS]/60 seconds, get an item from the list and write to a file with "save frames to movie".
I'm trying to code all this but there's some issues yet. Anyway @Jaymie , if you didn't propouse this is perhaps because there's something wrong that I'm not catching with this plan? Paul, could this schema solve you problem?