Hi Tim,
This might not be the answer you're looking for but I suspect this problem can largely be solved as a content issue more simply than waiting for an addition to L3DT's code to help you out.
It should be possible to create shapes like this using the existing height field editing tools (although I've never tried, and maybe there are some suggestions lurking there if you did try):
Given a low horzontal distance to allow for a nice curvature.
You'll also need to look at the material system a bit to add detail to the effect like the ripples show here:
Normal maps are supported so it could look pretty good with some nice textures applied.
If you want something automated then I guess one would write a plugin or script that simulates the fluid nature of sand, but I don't know much about dune formation so there would be a bit of research involved too. Sound like a complex problem (or one that could get more and more accurate as you worked on it) but a fun one.
You could go really insane and erode the sand as it comes into contact with water -- these guys managed this 'offline' task at 30Hz on an Xbox 360:
http://xboxygen.com/Medias/Videos-Xbox-360/Projet-Dust-Trailer-images-et, I don't know if you want to spend 2 years on it though