We have light group AOVs (for Surface shaders) in Arnold 5.
Super easy to use.

This is how you do it:

  1. Create some custom light AOVs (Render Settings -> AOVs -> Add Custom)
    Screenshot 2017-04-26 12.43.42
  2. Name it like this:
    Use the RGBA_ prefix and add some custom name to it. This is super important!
    For example: RGBA_lightgrp1, the second one: RGBA_lightgrp2
  3. Select a light and add to a specific light group:
    In the AOV Light Group section -> type in the name of light group without the RGBA_ part. For example: lightgrp2
    Screenshot 2017-04-26 12.44.18
  4. Do that for every light you have.
    You can add multiple lights to a light group of course.

That’s it.

By the way, I found that the “maximum of 8 different light AOVs are supported” is not true. 🙂 You can use more than 8. Maybe Solid Angle not updated the entire documentation yet.
The light groups in Nuke:
Screenshot 2017-04-26 13.46.29

Cheers, D