You should now be able to verify the form I shared above in the latest Collect beta using its XML: Filter select by distance.xml (1.4 KB). Central/pyxform will need to be updated before you can use the XLSForm.
I've also put together a form that describes basic reverse geocoding (going from a position to a place name), basic geofencing, and location filtering by distance (what we've been discussing previously in this thread): Distance examples.xml (7.5 KB) (XLSForm for future use)
Let me know whether you find the examples form helpful and whether there are other uses of the distance function that might be helpful to document.
Relatedly, the next Collect beta will include a geofence(point, shape) function that @Xiphware described at ODK geofence (v1) and contributed to Collect. It will let you verify whether a point is inside of a shape. Example form. This XLSForm will work in Central/pyxform today because of the if. I'll update once the next Collect beta is available.