Record ray arrival distances

[taktoa]

I'm interested in using this for sonar, where we don't just want the total amount of light (sound) that impinges on a given pixel, but the distribution of distances travelled from the light source to that pixel (which corresponds to the sound that would be received over time in each direction by the "camera" if all of the sound sources send out a quick chirp of sound). Would it be difficult to modify redner to support this?