Snowden called himself Verax[1]. Anyone who wants to send a message to Snowden* can just:
$ gpg --keyserver pgp.mit.edu --recv-keys 79DEBE35
$ gpg --encrypt --sign --armor --recipient 79DEBE35
and post it publicly; perhaps on Pastebin.[1]: http://www.washingtonpost.com/world/national-security/code-n...
*assuming you believe the key is authentic