Take a look at the sample from the form navigation thread. You'll see that there's a choice list with latitudes and longitudes for each selectable item. Those coordinates get pulled into calculates, as you described, and then you can do whatever you need to do with them. For example you could use a calculate to concatenate them with a space in between, make that field readonly and geopoint with the maps appearance. That way an enumerator could open the map and see the point but not modify it.
I really appreciate your detailed writeup, @J_D1. That gives me a much clearer sense of your needs.
This discussion is timely indeed as the Technical Steering Committee is now considering the general space of selecting existing map features as part of the data collection process. I tagged you in a related feature thread at Ability to load locations into the geowidget to guide data collection and also linked your writeup from ODK ecosystem entity-based data collection table of contents.
I'll try to keep this thread updated on our progress and may get in touch about doing a more formal user interview.