The easy way of changing to https looks to be adding a single character to the definition of $wgServer in LocalSettings.php.
$wgServer is used everywhere, I believe even for reader access, so I vote strongly against this change. ;-)
Normally the correct way of handling this is WebRequest::detectServer() which uses the protocol of the user's browsing state (if https, then https). I take this is a problem because the messages are decoupled between the event trigger and the recipient?
BTW, this sort of mediawiki discussion should probably be on a lis where much more experienced mediawiki devs can answer, so I'm moving it there.
That would change it in a few other places though, so sounds like 1 second of editing and 1 day of testing.
We could hack it in just for Echo, but that would either be hacky and specific to targeted installations or break on installs that don't have identical http and https urls.
BTW, my opinion is I'm okay with using http://... ($wgServer) right now for release and just capturing this request as a bug in bugzilla or in Trello for now.