We have made good progress on the implementation here. Given the level of interest, we have taken on several of the extensions described above. In particular, we are making the map appearance work with any select_one variant that specifies a geometry column. That means you could specify a choice list in the choices tab with a geometry column or attach a csv with a geometry column and use those in a select with map appearance. Note that this introduces a new convention: the geometry column is always used to map features when the map appearance is used. For the choices tab or a csv, the column value would be points in ODK format (lat lon alt acc) and eventually also lines and shapes in ODK format.
Because of the way we built that generalization, properties defined in the geojson file will be accessible in expressions the same way that values from other datasets are (I gave a useful example above).
In parallel, @Lindsay_Stevens_Au has added the ability to specify the property to use as the label in the parameters column of the XLSForm (e.g. label=the_label) and that just needs to be reviewed (PR).
Given what we've learned during our implementation, I propose one additional change: in the case of a select from an external geojson file, defaulting to the title property being used as the label. I think we'll want to support styling with the simple style spec sooner than later and so it makes sense to follow it minimally to start with (as opposed to using a name property as I'd previously described).
I look forward to any comments on the proposed expansions and changes I've described above. We still have to wire things together and do quality assurance. We hope to have a beta available by the end of the month.
Had a good conversation with @Ivangayton and @danbjoseph with some ideas on where we could use some geo-related help. In particular, they will look into offline vector tiles.