I'm not sure if this will work, but Bryce is an effective program for creating natural looking landscapes, and it does so by creating greyscale height maps on a mesh. In essence it takes the fractal generators and the adds in realistic erosion. You might be able to do some import/export with it. I'm not entirely sure, but its possible. At the moment, I believe the most recent version of Bryce is a free download. They tend to make their money on DLC.

Not sure if this helps or not....