Seconding this issue. A portfolio/project page takes time to create and curate. Having it go live as soon as it's created is unacceptable. The solution linked above by Jo_G does not work for this use case. I don't want to password protect my entire Portfolio collection, just the 1 new item that I'm adding to the existing collection.
Currently it looks like it's impossible to hide / password protect an individual collection item without resorting to CSS hacks.