You can do this with a Krakatoa CustomData render element. This exposes a simple, node based expression for determining what you want in your render element. Its pretty much exactly like a Magma modifier except it can only have one output (a color) and the particles are all in worldspace already. If you are trying to render worldspace XYZ its as simple as creating an InputChannel node loading the Position channel, and connecting that to an Output node. Like so:
If you need camera-space relative values you can use a ToCamera node in between. You might find that volumetric data (like that rendered by Krakatoa) doesn’t work particularly well when it comes to render elements like Z-Depth or XYZ. That’s the nature of semi-transparent media, since its impossible to pick one value that represents the entire volume along a pixel ray. Deep images like those supported in OpenEXR 2.0 should help solve this problem in the future.