Hi, I really like this idea! It would solve some use cases for me, where I was waiting for a better solution than the individual file.license files (e.g. folders with many icons, folders with test data). And I think that it is important that the file is easy to see, meaning it is not a hidden file. Same for the fact that it is easy to read by a human.
The only point I am a little bit undecided about is, if one should really allow arbitrary regular expressions in the REUSE.yaml files, especially going outside of the scope of the folder the file is stored in. This will make it harder to detect issues with contradicting license statements. However, the reuse lint tooling could be the answer for this.
Best regards, Andreas