Here is a quickie I did with a simply deformed sphere, a spline mushroom base object and a surface replicator. You can see the source object to the right of the cloud. The spline object has it's visibility turned off. You can expand the image if you want to see more.

This could be enhanced with a little more work on the alpha channel in the shader and possibly even a displacement map. In fact, there's a couple of more things that could be done with extra replicators and maps to control the replication that could add some cool detail. 

The problem with volumetric clouds is the render time and overlap issues where the clouds will render black instead of seamlessly merging. 

Enough procrastinating, back to work...