So there are two use cases where a registered user would post as AM, you say:

E.g., when forgetting to log-in

Just as an example, in some other forums you can and have to enter a guest nick name. If you are a registered user and your session timed out and you start a reply, you will be noticed that you are guest and have to enter a guest nick name. So it's impossible to "forget" to log in.

Maybe something similar could be introduced here, an extra checkbox and/or a clear "warning" that you are about to post as AM. This can be implemented probably very easily.

being at $work, etc.

Why would one not want to log in when being at work?

It should be possible to implement this RFC without introducing changes to the database layout. The unlock-phrase could be created by hashing (e.g. MD5) a secret initial vector + the node-content + the node-ID + some other unique node-specific value.

The node content, ID and creation date are public (*). Where does the secret initial vector come from? There has to be something secret in the database which you can compare to the unlock phrase. I don't know the db schema, so I'm interested in how one could do that without changing the schema.

(*) Well, the rendered node content is public; I don't know if there is an url to view the raw source of a node. In other forums you usually can get the source by replying with quoting (which I consider very useful, but I'm getting off topic).
update: the xml view seems to display the raw node content. update 2: without the newlines, though

update 3: the only use case I could think of is posting from a public, untrusted computer with a keylogger, and that's certainly an interesting one.