Is it possible to limit fog (the kind generated from the render menu) to certain regions of the viewport?
Yes, there is Fog Alpha and Fog Texture in the fog settings. next to the colors. Play with the Fog Alpha and you see what it does for you. Fog Texture is fun, but I have not found a good use for it. Go with the Alpha and you can use it mask the impact of fog and even create clouds. You can mrgbz an alpha of your document, then edit that as grey scale in PS to be very precise…
Cheers
Lemo
Yeah, I was thinking to use the alpha for something like that. Being a lazy sot though I was hoping for a one step process. :lol:
Sorry… can;t help with that. You have to live with the complicated two step process 
Lemo
…don’t make the trick?
Stencils might help to manually paint a fog-like effect, but they cannot be used to control rendered fog. Since both rendered fog control and the stencil are derived from an alpha, it’s kind of pointless to use the stencil for this. It would actually be more work!
Are you sure ZBrush is the program for you? 
I take you’ve seen my recent posts then huh?
Seriously though, Lemo had the right idea, when I was looking how the fog interacts with some alphas it occurred to me how easy it would be to make clouds if you could select a limited region in which to render the fog.