Fire/Smoke Volumetric Custom AOV

Hi, I see that a mantaflow domain has some interesting attibutes that I can call in the shader editor via "Volume info" or "Attribute" nodes.

If I connect, for example, the attribute "flame" to the Volume input of the Material Output I get a nice "alpha channel" of the flame. But, here comes the question, how can I get it out as a custom AOV?

If I create a custom AOV nothig outs, only a black image.

Obviously I can connect the desired attribute to the volumetric output (as the image attached) and render several times, but I would like to get it with just a single scene.

Anyone knows a way to get those AOVs in a single multylayer exr render?

Thanks in advance.

Cheers, Ignasi.