Is it possible to re render a single pass and not the whole scene again? I'm experimenting with the values for the mist in my scene (Cycles), however each time I want to test the new values I can't find a way to only render the mist pass. This results in the whole scene being rendered again which takes a lot of time and makes experimenting with the values a real drag.

So the question is, can I render only the mist pass and then use it with the original render in the compositor?
It appears that the answer is no, however I'm not sure why this wouldn't be possible for some passes such as Object Index, Material Index, etc.)

As a workaround, render only one sample (you can also render at a lower resolution while testing). 

To combine with the original render you will have to save the render to the hard drive (you may want to use exr to preserve other passes) and then load it back into the compositor with an Image node.