Pseudonym definitely applies to people other than authors. Online users, movie stars checking into hotels under another name for security, victims of crime the media aren't naming, and so on all count as pseudonyms.
For a project or activity, and for products at some stage of development, I think code name is a better choice. For example, the Doctor Who tapes were labelled Torchwood during the series reboot a few years ago, to lessen the chances they would be stolen and leaked. (Later the series that spun off from Doctor Who was called Torchwood as an inside joke. The names are anagrams.) This could also apply to a book while it is being written.
I'm not sure there's a word in English for consistently calling something other than a person by a different name, like using "The Scottish Play" for MacBeth. There's nickname but it covers a lot of different uses.