Getting reflection depth for planar mirrors

[genesys]

I would like to add some planar mirror objects with perfect reflectivity to a scene but instead of getting the distance to the mirror surface per pixel in the depth buffer, I would like to get the total depth of the reflection (ie. the distance the ray traveled from camera to mirror surface plus the distance from the mirror surface to the primary intersection of the reflection ray.
How can I do that?

[maru]

Why do you need this? Can you share an example where this would be useful?

[genesys]

I'm creating simulation data for machine learning and need exact reflection depths. But I think i figrued out how - seems using propagate masks this should be possible https://coronarenderer.freshdesk.com/support/solutions/articles/12000066916-how-to-use-masks-with-reflection-refraction