The WikidataPageBanner api allows editors to create pages with a banner using PAGEBANNER magic word.
The information can be accessed via: https://en.wikivoyage.org/wiki/Special:ApiSandbox#action=query&format=json&prop=pageprops&titles=San+Francisco&ppprop=wpb_banner%7Cwpb_banner_focus_x%7Cwpb_banner_focus_y
It would be helpful to have an api or a framework in core to allow a shortcut for setting of these parameters without having to edit the underlying wikitext as then the iOS app could work out the correct values and save for wpb_banner_focus_x and wpb_banner_focus_y.