Here is another way of working with comments mentioned by some internal Adobe community members:
Another view on this question would be to POST the comment to the asset path. For instance, to a comment to this mountain picture that is part of the Geometrixx samples
POST to: http://localhost:4502/content/dam/geometrixx-outdoors/activities/snowboarding/PDP_1_c05.jpg
With the following request attributes:
1. :operation:
granite:comment
2. path:
/content/dam/geometrixx-outdoors/activities/snowboarding/PDP_1_c05.jpg
3. message:
Posting a comment programmatically....
This results in a new Granite comment on the asset.
[img]AComment.png[/img]