Let me start by addressing a more general question: How to promote some kind of behavior? How to draw attention of the users to some particular issue? (Be it related to tagging, formatting posts, closing, reopening, approach to new users, etc.)
Some things I can think of are:
- Lead by example. (This is probably the best one.)
- Make a post on meta. (Well, some users do not read meta; but you might reach at least some of them. If your post on meta is sufficiently scored or if the moderators decide to make it featured, then it gets into community-bulletin and it will be visible to more users. If the issue is rather important, it could also get into faq.)
- You can create a comment template. (It is easier to copy and paste a comment rather than type it every time. If making a comment about particular issue is made easier, more users will post such comments. If there are more comments about this issue, more users will read such comments and become aware of the problem.)
- If it is suitable for that particular issue, use chat. (You can mention the issue in the main chatroom. In some cases, it might be also suitable to create a separate chat room. This happened in the case of ongoing effort to reduce the number of unanswered questions.)
- We also have a blog, which can be used for rather serious issues related to this site. (But it is intended to be mostly for posts about mathematics.)
- The issue could be mentioned in help, site tour or some other suitable place.
- There are automated messages and comments added either by software or by bots.
Now to the specific issue of comments related to closures:
- Many users are posting such comments. For various reasons, some users do not want to post such comments when closing questions. But it does not matter that much whether the comment (with an explanation what to do if a question is put on hold) is posted by one of the users who closed the question or by somebody else. Users who care a lot about this might find questions recently put on hold using review history or 10k+ tools and add comments where needed.
- I think that the post I linked in my first comment is doing exactly what you asked for - it urges users to leave helpful comments on closed (on hold) questions.
- I think that there already are some comment templates which are suitable for this situation. But if you have another take on this, you can contribute to the list of comment templates.
- I am not sure to which extent chat would be helpful in connection with this particular problem. (A separate chat room would probably not be suitable for this particular problem.)
- For this particular issue, using MSE blog is not the way to go. I also do not think that this should be mentioned in help. (Especially since there is no clear consensus.)
- I doubt that posting comments on closed/on hold questions could be in some reasonable way automated. (Posting comments on recently posted question is somewhat different, since the OP is likely still around and the comment can easily be deleted after short period of time.) But we do have close reasons which give an explanation why the post was closed. (As far as I know, they are customizable to some extent. But before changing any of them, some kind of consensus in the community has to be reached.)