generative modeling for Rhino
OK, found it.
Must have been a late-night thinking barrier.
The answer was too close to see (as suspected).
Describing the problem helped clarify what I was after ;-)
Path Mapper (A;B ->B;A)
Next time I will post this on a local grasshopper3d.com-dummy and sleep it over before posting for real ;-)
i will go out and buy one of these ducks this afternoon ;-)
Thanks for the link,