Just for your information, I tried the method you described. It didn’t work quite as I was expecting but still, it ended up well. I just selected the original sign and brought it forward (on top and at the bottom of the newly created shadow), played with a gradient shadow and now it looks good enough for me.
Thanks for your help and for this plugin. It’s a great time saver for isometric projections from Illustrator. Designs are lightweight ( in comparison to a native 3D plugin), and most importantly editable both in color and gradient. If you are into the printing business it is simple to create one layered gradient which makes printing and prepress a breeze.