From Wiki:WikiMail: A message left on someone’s HomePage, preferably in the Wiki:WikiMailBox. More effective with some people than others.
It doesn’t seem unreasonable or impolite to say, “Please don’t leave WikiMail; send email instead.” if you aren’t a Wiki:RecentChangesJunkie or frequent visitor to your own page.
Also note that EmacsWiki has Talk pages where people can leave comments.