Ah, right, so you didn't draw that brick structure in Viewingdale.
Hmm, 'use' might be too strong a term. 'poke about with' might be a better description of my relationship with Gimp.
I think it would be difficult to add shadows to a Viewingdale map (other than by exporting a 'dead' copy to Gimp and using the drop shadow feature to 'finish' it as you suggest.
Unfortunately, my primary purpose in mapping isn't to create a work of art to publish, but to create a virtual game world in Viewingdale, so if the shadows aren't in the zoomable 'live' copy, they're just wasted effort from my POV. And even if you can get them in a 'live' drawing, I'm just not sure it's worth the effort for my purposes.
Your adaptation of my drawing is a start, but maybe a discussion of how shadows might be doable in Viewingdale should be taken back to the Viewingdale site to your thread or mine, or at least to a different thread here if it will be of interest to others.
However, my initial thoughts are:
You could maybe take a rectangular transparent grey fade and make a fixed size icon of it, but as I understand it from my blog correspondence with Redrobes the other day, you'd need to make separate shadow icons for every wall type. You'll have trouble with walls that meet at anything other than right angles, and you'd need different shadowlengths to suggest different heights of wall and the only way to do that is to make more icons, you can't lengthen a shadow 'in drawing'.
Maybe Gimp or Inkscape could be used to make a drop shadow around an object (say a grey rectangle, circle, inner and outer corner, etc) then delete the shape, leaving just the shadow. That could then be imported as a png image in Viewingdale, and should be interpreted as a transparency.
Unfortunately, my first attempt with shadows in Viewingdale was a failure: the dais in the throne room of my castle.
As I described in my blog somewhere, as you enlarge the steps, you also enlarge the shadow and this looks wrong because the shadow radius is proportional to the dais radius in a drawing, but proportional to the step height in reality. Consequently, the three concentric steps of the dais look like they're different heights as well as different radii.
(It's too small to see in the picture here, but there's a detail in my blog.)
You'd get the same problems with wall shadows. Almost every wall would need to have its own shadow icon generated, since they'd differ depending on the wall height.
It looks like a can of worms to me.