Hi Jerome,
Sorry for the slow reply - the main tricky bit with comments is to be sure you're not accidentally disclosing personally identifiable information (especially of minors) in public.
We didn't define a comments paradata schema b/c we didn't want people to make that mistake. That said, if you can manage for PII, LR seems like a great system to interchange comments. Some things to get into place:
1) A terms of service agreement with your users that permits you to publish their comments in public. Twitter for example does this, and they seem to be doing ok! Amazon comments also are public, so it's very do-able. You might look at their comment ToS to see what they say about posts from 13-18 year olds (under 13 are in a different category altogether).
2) Encoding that ToS into the LR comment paradata you submit. So the license on the comment stays the same as when it was on your site.
With all the said, I think your paradata format looks good for comments - so barring other input, you should consider it definitive. If you'd be willing to update our paradata spec doc, that would be great to have it in there (but with the PII caveats above clearly stated).
Anyway I hope this input is helpful. All feedback/thoughts (and comments!) welcome,
Steve