Back edges are a raster thing to begin with which is why they only show when you render in Raster. One option that I find works well is to leave the viewport in Raster for the time being and then trace with the line tool over the back edges. Those lines can be set to dashed or dotted or whatever works. You can adjust the line weight and color if you wish. When you're finished with the back edges in the viewport, change to Vector rendering. Frankly, this is a good option for me because I have more choices for the sort of lines I'm showing. I generally put all those lines I'm drawing in LO on a layer higher in the list than the viewports.
