This is the entire point of the Stake your Bitcoin Address thread here:
https://bitcointalk.org/index.php?topic=996318.0 except instead of PMing the bitcoin address to a newbie, your address and a signed message is posted publicly, quoted for records, and verified by multiple trusted members of the community.
That thread you linked to requires a high level of computer literacy to participate. I don't want to learn how to sign messages, all I want is something fast and simple I can do to help me recover my account if its hacked.
One of theymos's posts says that emails are never deleted unless both the sender and all recipients delete them. Additionally he says emails are sometimes stored long-term in the forum's periodic database backups. Sending someone a pm is good enough for me, providing they agree not to delete it.
That code snippet has to do with deleted posts, not PMs.
PMs are deleted from the database when the sender and all recipients delete it. If this doesn't happen, then it's not deleted. There is no expiration. PMs can also end up stored long-term in the forum's periodic database backups.
On this forum, topics and replies are never truly deleted, and edits are also logged.