Both ravells and myself have tried a similar technique. See here. Worked pretty well.

My experiments stranded when I tried to include automatic shadowing. I needed two brushes, one with the houses, and one with the "height maps", but with excatly the same settings (dispersion, rotation, scaling etc), which proved to be next to impossible to achieve, as in Xara, all brush settings are set using tiny sliders.

I used the same trick on the trees in this map.

Fransie