proper hook for message body transform?

[Eric Abrahamsen <eric@ericabrahamsen.net>]

I hope someone can give me a little advice on a project I'm working on.
I'm trying to put tags into a message body that will "do things" to my
org-mode setup -- automatically create TODOs from sent messages, to be
specific. What I'd like to do is have a little text tag that sits in the
message composition buffer, and when the message is sent the tag itself
is stripped from the message, and the good stuff happens in a hook.

I tried looking at the mml package to see how they do it, but can't for
the life of me figure out which hook the mml functions are attached to:
the only hooks I see in the source code are those that are used to kill
the tags if the message is discarded (I think).

Does anyone have any words of wisdom for putting things in message
buffers that are then stripped (and other functions triggered) when the
message is sent? Neither `message-send-hook' nor
`message-send-mail-hook' has anything attached to it by default, which
makes me think there's some better way of doing this. Anyway, any
experience shared would be much appreciated!

Eric