Oh, that's easy enough to do right now! It's not part of the game, but if you go to the folder:
- C:\Users\<Your Name>\AppData\LocalLow\Team Connect 4\NeuroNovel\NaninovelData\Saves
All you need to do is rename the file called GlobalSave.nson to something else (or just delete it if you want). Then it won't remember which endings you've seen.