A Distance Shader as the Mask for your Density Shader (or Final Density Modulator) should work fine, although it would only create a spherical "hollow" inside the cloud, not one the exact shape of the exterior but shrunken, so to speak. But what exactly are you wanting to achieve? If you're in the center of a hollow cloud you'll just see... gray around you, right? And if you're seeing it from the outside, you wouldn't even know it's hollow, except maybe by odd lighting interactions, right?
- Oshyan