Didn't get any replies on that one so could I suggest a painting facility is added to GG. I make models for various simulators including Train Sim 2015 and they have a great facility for painting any texture straight on to the ground. With GGs ability already built in it should not be a marathon to extend this to full user painting.
Anyway for now thanks to some free models by GTOX I have pinched his idea and made some ground tiles as shown below. They are quite memory efficient so thanks for the heads up GTOX.