Just to follow up to my own question, 🙂
I was able to chat with support, who confirmed that importing and georeferencing a standard raster (an image file, not a 3D file) in a 3D map is not possible, at least directly. The workaround I found was to import the raster into the 2D map, georeference it, and then copy it over to the 3D map. Works fine.