I was looking at the options for Clamp, and it seems like it could just as easily be applied after rendering rather than having it 'baked in' to the render - in the same way that exposure and the upcoming denoise works. I know exposure actually happens before the screen-space colour management transform, so I guess it is more like the denoise option, which has the ability to render it and change the settings after the fact.

I'm not sure of the math and decisions made by the clamp settings, but if it could still work on the data remaining after the render (rather than on each sample) then it would be great to have a 'post-processing' option for clamping. It would allow for a much more responsive and interactive way of getting rid of fireflies without losing too much realism and detail.

I think a check box like in the denoise panel to switch it to a 'post' effect would be an effective way to implement this, but it depends on whether this is possible in the first place.