Can I write on a comment on SO:
you can insert the link of the new question here (on the comments) if you want my attention there?
As noted on the comment privilege description page, it's appropriate to use comments to:
- Add relevant but minor or transient information to a post (e.g. a link to a related question, or an alert to the author that the question has been updated). [Emphasis added]
Usually that's a comment on a question pointing the asker towards an existing potential solution, but indicating that an answerer might be interested in a related question is okay. (It is not okay to pester someone who answered one question to also answer another, but if you genuinely think they (and future visitors) might be interested in the related question, that's fine.)
As such, proactively adding a comment saying, "Please let me know if you have any other questions," isn't forbidden, but isn't exactly what they're intended for. If you want to provide continuing support to a questioner, it's better to have them contact you via email, chat, or some other channel. That said, I don't think that anyone would be upset with you for going above and beyond to help someone.
In any case, delete any obsolete comments that won't be useful to future visitors.