This may be a little past its prime… 
But depending on how many of your users have turned off their email, a new #announcements style category where you set it to be automatically Watching First Post may do the trick. If you create a topic in there, then an email should go out to everyone who doesn’t have their email set to ‘never’. (and then you can retire the category/remove it from default Watching First Post after it’s served its purpose)
Out of curiosity, how did you handle it in the end?