Similar to user talk edit notices, Conpherence should allow me to add a fat warning to be seen by anyone attempting to add me as Conpherence target. Such notices are typically used to indicate favourite contact venues or other tips for efficacy of communication with the user.
As far as I know there are no plans in upstream to implement this and we don't plan to have a downstream patch for this either.
Maybe a cleaner approach would be yet another per-user setting, but thinking about that it would be required for every single application in Phabricator? If I use Phabricator in some areas but I do not plan/want to use Maniphest, I cannot express that either currently. Too much effort, too little gain.