The Author Reply is nothing more than a SINGLE FIELD within the Rating record itself.
What YOU want is an entire SYSTEM that has its on REPLY Table that will store RECORDS for EACH REPLY (just like Comment Replies). A Controller with actions to Create Reply, Save Reply, Edit Reply, Save the Edit, Delete Reply, Like Report, Report Reply, Warn Reply, several methods added to the Review Model to handle all the fetching/preparing and permissions for all those new functions, DataWriter to handle the new table for insert/update operations, Handlers (like, Newsfeed, alert, spam, search, moderationqueue, moderator log etc), phrases (a ton of them), templates (dozens of them to handle Views, newsfeeds, alerts etc).
So ya, its MORE than just a Permission as 99% of whats needed isn't there.