I'm using :0fbw to add a line to the body of the message before saving it.
echo "message" > .file (ahead of time)
:0fbw
| cat .file -
(Suggestions for better command? Particularly one that doesn't need an
external file?)
However if the message is sent in mime format the extra text is not
visible since it's hidden inside the mime stuff. So it would be nice if
formail could parse the mime stuff, find the first body of type text/plain
and then either run a program on the text, capture the output, and put
it back into the mime message, or allow adding lines to the text, then
sending it to stdout.
When finding the mime part, it should take a parameter of the content-type
you are looking for and then the sequence number, to find the frist,
second, etc part of the message with the type (default text/plain, 0).
As for built in text changing actions, I would add functionality to add
text at the top, after x line, at the bottom, x line from bottom. And the
functionality that head and tail have. That should be enough for most
users, with other stuff done by using the pipe through program option.
-Ariel