Only normalize "default" track?

[georgev93]

I would like to only compress one track of my input mkv. In my case, it's an episode of a show with one AC3 5.1 track (marked with the "default" flag) and two commentary tracks. Of course, I don't need to normalize the commentary.

So the input track configuration of my mkv is:

1. AC3 5.1 (marked as "default")
2. AC3 Stereo Commentary 1
3. AC3 Stereo Commentary 2

And I want the output track configuration to be:

1. AC3 Stereo (loudnorm and stereo version of input track 1) (marked as "default")
2. AC3 5.1 (input track 1, now no longer "default")
3. AC3 Stereo Commentary 1
4. AC3 Stereo Commentary 2

(I threw in a "convert from AC3 5.1 to AC3 Stereo" before loudnorm, which is a separate thing that I know how to handle. I just want to give a complete picture)

So what I'm asking: can I tell ffmpeg-normalize to only loudnorm the default track? If not, can I tell it to only loudnorm the first track, or a track that I select? If not, I'm happy to throw in a PR if this seems like a useful feature.