The other option is less useful for mapping but might have some artistic uses...

You can use a map with transparency, and it will use the alpha as the density map, and the colour as the colour to paint with.

Here is such a density map:
Click image for larger version. 

Name:	map2.png 
Views:	304 
Size:	124.9 KB 
ID:	4994

and the result, using the hex brush again:
Click image for larger version. 

Name:	map2_result.png 
Views:	234 
Size:	26.6 KB 
ID:	4995

Enjoy!

-Rob A>