You don't securely hide anything. You send it using any public channel that happens to be convenient and pre-agreed - YouTube, Vimeo, some squirty porn site, or whatever.

The one thing you don't do is try to hide it.

What makes it invisible is the fact that it looks just like the rest of the content it's hiding in, and doesn't come with a tag that says 'SUPER SEKRIT HIDDEN CONTENT - PLS TO NOT DECRYPT THX'

Which is the obvious problem with Tor and PGP email.

Obviously this doesn't work for simple emails. But there's no reason in principle content piggy-backing couldn't be added to any publicly accessible content distribution system, and the packaging and unpackaging couldn't be automated.