Great idea, looks great, man.
I never, ever would have thought of this.
Not sure what the render holdup is here, but this example of your scene took 2 hours to brew.
I did, however, swap it over to the path tracer, detail and .6, AA at 4, Erosion enabled.
I raised the light a tad to show the cloud gag.
Great idea
On the second frame (much smaller and faster to render) you can see how the mist hugs the terrain, plus the flow map is taken directly from the erosion plug-in and piped into the texture mask to bring that out
47 minutes with the standard renderer