I am just placing this in case someone wants a non plugin way, if you enable a simple MySQL trigger you can do the same.
On the users table add the trigger on INSERT (you can do this in most database editors like Sequel Pro really easily). Just add the following.
Action time: BEFORE
Event: INSERT
SET new.preferences = '{"notify_discussionRenamed_alert":true,"notify_postLiked_alert":true,"notify_discussionLocked_alert":true,"notify_postMentioned_alert":true,"notify_postMentioned_email":true,"notify_userMentioned_alert":true,"notify_userMentioned_email":true,"notify_newPost_alert":true,"notify_newPost_email":true,"followAfterReply":true,"discloseOnline":true,"indexProfile":true,"locale":null}'
You can also use
CREATE TRIGGER users_insert BEFORE INSERT ON `users` FOR EACH ROW SET new.preferences = '{"notify_discussionRenamed_alert":true,"notify_postLiked_alert":true,"notify_discussionLocked_alert":true,"notify_postMentioned_alert":true,"notify_postMentioned_email":true,"notify_userMentioned_alert":true,"notify_userMentioned_email":true,"notify_newPost_alert":true,"notify_newPost_email":true,"followAfterReply":true,"discloseOnline":true,"indexProfile":true,"locale":null}';
To set the new rule using a query command.