Kadri, Was, appreciate the quick responses.
Turns out I was pretty close . . . but I have a feeling I would have been at it for a while yet. Never would have considered using a twist and shear shader as the warper, but it works great.
In your first file, I noticed that with some seeds the cloud also appears outside of the masked area and occasionally completely obscures it. So I borrowed from my earlier attempt and piped the ps into the density slot and the sss into the final density slot, sending both through warpers warped by the same twist and shear shader. This is pretty much what I was looking for, so thanks for breaking my mental logjam.
Jordan, thanks for the share. That's an interesting file and there are a few things to parse there. Out of curiosity (I haven't tried this) would a distribution shader work as well in place of the surface layer?