Have you tried searching for a cloud brush set on teh googles? There are some really great sets out there (mostly for photoshop) that you can quickly build whatever you need. I'm not an inkscaper yet so I'm not a ton of help but if you have PS you can get something going pretty easily.
I mainly use paint.net. I bought corel paintshop pro, but haven't learned how to use it yet.
There is a tutorial for paint.net, but I'd probably spend a couple hours installing the effects and then following the tutorial.
If someone has some PNGs, I could throw together what I want to accomplish in about 10 minutes.
I found one that looks like exactly what I want searching google pics, but it takes me to some kind of crazy redirect website instead of loading the pic.
Found these:
http://scri8e.com/stars/PNG_Clouds/Not as transparent or whispy as I'd like, but I can probably modify them well enough.
Maybe I should be searching for fog?

EDIT: According to a youtube tutorial, there is a "cloud" effect in paint.net that makes clouds.
