What's the best way to auto-sign some emails but not all?

Is there a nice way to always sign emails based on e.g. the current From
address?  I don't want to sign everything, but I would like to sign all
emails I send from one specific address.

I'm using gnus-posting-styles to automatically set up things like from
address, default headers and signature based on context.  But I can't
figure out how to enable automatic signing.  A simple approach like
adding

 (body "<#secure method=pgpmime mode=sign>")

to an existing posting style rule sort of works.  But it fails on
replies with quoted original text.  The body text end up between the
quoted text and the signature, which obviously makes sense for any real
body.  But the mml tag is not recognised and is sent verbatim.

I tried adding the tag in more complicated ways, using posting style
eval and a function inserting the mml tag on the first line of the body.
But the message buffer is always empty when this runs, so that doesn't
work any better.  The tag still ends up the same place.

I guess I'm looking at this the wrong way.  But I'm stuck.

(My elisp skills are extremely limited, so I didn't want to re-implement
the posting style context matching in a sending hook or similar.  That's
way out of my league)


Bjørn