Hi Cynthia,
Very valid feature indeed.
I was also trying to think through another topic we have under discussion and which is linked, is of comparision between Google maps vs Openstreetmaps (http://community.openfoodnetwork.org/t/open-street-map-vs-google-maps/492/2
As the base of the optimization feature you are discussing is based on the underlying map data, and hence it becomes important to take a call on too, to build on it.
Mapbox the community I know an out of the box feature called Directions API , which you would want to look and see it addresses at what we are discussing here.
The tech. involved also are Javascript Libraries , so @maikel can have a look and see how easy it is to accomodate.
Also I was checking on the list of Opensource options you mentioned like Opendoorlogistics, Graphopper etc, and most of them seem to be working out of the box with OpenStreetMaps, and have been built on it.
Hope I didn’t hijack the topic, but I kind of felt both the discussions were connected 
Cheers.