Oh shoot, you're totally right -- doing a quick search reveals that Blender's Cycles engine has totally had this feature for a while and I was just completely unaware of it despite using it frequently! Although even without the heatmap pass being unique/new I'm still super excited for the rest of MoonRay, especially those delicious volumetrics!
Oh yes, I'm not knocking the release of MoonRay as a whole. I was at HPG '17 when the paper that erichocean mentioned was present. I'm definitely excited to get a peek at it.
I also wonder now how much work it was to disentangle it from their infrastructure? A lot of internal studio tools and libraries tend not to work outside of their studio's intranet -- not so much from direct copy protection, but more just because of heavy assumptions about filesystems and servers.
https://docs.blender.org/manual/en/latest/render/layers/pass...