Clever idea, Rob! The mosaic tiles do not look bad in the GIMP edition you've posted; not bad at all. I think one would still need to hand-draw in most of the same alley & sidestreet lines, so I'm not convinced this is a huge time saver. Is that assumption correct?

Also, after playing with Photoshop a bit I can't seem to reproduce your exact effect, even using the mosaic, mosaic tiles, and other pixelate/texture filters; even with some play adding other filters, etc., the result appears too "geometric" in my tests.

Another option occurred to me while playing: creating a "buildings" brush that dabs in groups (or lines) of houses; I had thought of this more for villages, where the spaces would be greater, but I wonder if I could make it work for cities, too. I'll have to experiment with that.

Thanks again, and nice work!