A first version of this functionality is now in beta! XLSForm support is not ready yet and we have decided not to do the dashed line for now. For now it only works in Google Maps.
Here's a demo XForm: ref_geo_demo.xml (3.3 KB) You don't need to open this or understand it -- if you upload it to Central it will let you create new Entities with geometry that will then be shown the next time you fill out the same form.
The first section of the form shows using an Entity List as reference for capturing new geometry. The second part shows using a repeat for the same.
Please let us know what you think!