Allow snapping to vector map image layer in web map

[Matt_Trebesch]

Give ability to snap a hosted feature layer to a "map image layer" of vector data in a editable web map (or web map application)

Example - some data is published as "read only" map images layers.  

I publish a read-only service of Parcel Polygons, which are vectors.

Would be great if a corresponding hosted feature layer (or view) could snap to those parcel polygons in editing map.  Yes, I know I could make another hosted feature layer of parcels, but it would be great if I could recycle an existing map image layer...  and therefor help my editors keep better data.