If you can post to LiveJournal with PGP, can you post over the Mixmasternetwork and use LJ as your own invisiblog? I was going to say I don't see anything about yielding a proper From to match your allowed_sender@example.com from LJ's help on the feature, but then I noticed the PGP example shows posting from anywhere@example.com instead. So I guess so!
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If you do need a valid address to sign up somewhere like LiveJournal, you might be able to use http://spamhole.com/ to create a temporary forwarding address that is deleted after a short time period.