Attaching random content to emails reminds me of Emacs's 'M-x spook' command (and I'm not even an Emacs user).

As for distinguishing random data from PGP that is very easy: PGP messages (even binary ones) have some headers that determine who the recipient is, what encryption is used, etc. See here for the format of PGP messages: https://tools.ietf.org/html/rfc4880

What you could do is to generate a public/private PGP keypair, then destroy the private key, and encrypt your /dev/urandom data using that public key.