Arnold 4.2.12.2 added support for per-light AOVs for volumetrics.

Here’s how to set up per-light AOVs in MtoA 1.2.6.0:

1. In the light Attribute Editor, enter a name for the light group.
2. Create a custom AOV for the light group. Give the AOV a name that starts with “volume_”. For example, if the light group name is “red”, then the AOV name is “volume_red”.