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.
In that case one could create an extension that creates a PGP keypair _for the recipient_, attaches encrypted crap to the e-mail, and deletes the private key. Is that what you mean?
The idea is to add by default an encrypted message to all e-mails that contain nothing useful, to make it indistinguishable from e-mails that do contain useful encrypted stuff. Is that possible? Does it make any sense?
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.