No, the answer in the end hinges on the fact tat I developed the ComboBox widget and accompanying class, so I can do what I want with it. One thing I can do is pass the ComboBox's TextView to the OnItemSelected callback, instead of the item selected from the list which is the default. If I can find out the TextView in this way, I can get its ViewHolder and from there to the data held in the specific ComboBox. After that, it all falls into place.
Su substantial progress today. I can set up a list of Waypoints associated with a particular Route, populate it, define the order of Waypoints, and work out the distance and bearing from each Wayoint to the next and then display the total number of Waypoints in the Route an the total distance to travel. All good stuff. One or two more things to do with Routes, such as the ability to delete a Waypoint from a Route, and I can then start deploying this technology to the other main functions in PCDB Waypoints.
Then on to the really important things. Updating my charts before our voyage to the Antrim coast in Northern Ireland in Caledonia this summer!