I'm currently working on an update for the Steam release, which will add the option to reveal hidden achievements in the New Game+, among other things (another player suggested this idea, which I thought was great) . This should be available in late July.
However, if you don't want to wait that long, I can post the hidden achievement names and descriptions here, if you don't mind editing the post title to include the word "Spoiler" (or I can do it), just to warn other players. Just let me know!