Hmmm... I had the logic backwards, of course. I settled in the end for a mocomp to the point 42% ahead of each source frame, (using MFlowInter) and then multiple Decimate calls.
It sort of works, but still isn't great. (E.g. static sequences get artificially shortened in favour of keeping all the frames with significant movement.)
Looks like I have another filter-hacking project to play with, then...
(Maybe one day I'll get something working well enough to publish here!)