Hi guys, This should already be working. Here are some things to consider...
- The image must be true B&W. (The ID mask only shows the decal transparent in places where it has 100% transparency)
- Enable ''Clip'' in the opacity channel.
- In the decal object, ensure ''visible in masks'' is enabled.
- In the ID pass, set it to ''Instance'' or ''Object''.
- Render ;)
Hope that helps!