OK, I've got the answer:
In order for cover fills to show up for roofs and morphs (and maybe some other types of elements...who knows?), the storey's cut plane has to be above the vertical location of the element (which explains why the overhead linetype was showing).
However, it appears this is not the case for slabs. Grrr... yet another example of the frustratingly inconsistent behaviour of different element types! (I do wish GS would stop and spend more time fixing what they've already half-built!)
In my case, where I have all the terrain and landscape modelling on one sub-floor & landscape storey (which, on a sloping property, has elements that can vary in location from below the ground floor to above the roof height) , setting the FPCP to above the roof storey solved my problem.
How to Change the FPCP:
To change the Cut Plane for the current storey, go to the menu: Document > Floor Plan Cut Plane....
If, like me, you want to set the cut plane height to many storeys above the current one, you'll need to add up the storey heights found in the menu: Design > Storey Settings....