KNotify allows specifying different notifications for the same event based on different "Contexts" (a stringlist passed to the KNotify arg) The Text-UI supports this, but there is no method to set these per user notifications. I think this should be in the context menu of a contact. Maybe it should be edit-able from the text-ui application too. For help you will need to check the telepathy-chat-handler code (and possibly update it if it's wrong (as it has been untested)). Kopete code will be a good reference on how to load the correct notifications dialog.
This is currently ongoing and in review.
Has been fixed via multiple review requests and merged into master