I don't think that this hook can be used.  Let's have a look at the
following message which conforms to RFC 2015:

----------------------------------------------------------------------
Date: Sun, 13 Jun 1999 13:32:15 +0200
From: mutt@cygnus.stuttgart.netsurf.de
To: Florian Weimer <fw@deneb.cygnus.stuttgart.netsurf.de>
Subject: Signed with attachment
X-Gnus-Mail-Source: directory:~/Mail/INCOMING/
Message-ID: <19990613133215.B2917@deneb.cygnus.stuttgart.netsurf.de>
Mime-Version: 1.0
Content-Type: multipart/signed; boundary=YD3LsXFS42OYHhNZ; micalg=pgp-md5;
	protocol="application/pgp-signature"
X-Mailer: Mutt 0.95.1i
Lines: 50
Xref: deneb.cygnus.stuttgart.netsurf.de misc:178


--YD3LsXFS42OYHhNZ
Content-Type: multipart/mixed; boundary=ADZbWkCsHQ7r3kzd


--ADZbWkCsHQ7r3kzd
Content-Type: text/plain; charset=us-ascii
Content-Description: Message

A signed message with an attachment.


--ADZbWkCsHQ7r3kzd
Content-Type: text/plain; charset=us-ascii
Content-Description: Assembler code
Content-Disposition: attachment; filename="t.s"

	.file	"t.c"
	.version	"01.01"
gcc2_compiled.:
.text
	.align 4
.globl foo
	.type	 foo,@function
foo:
	pushl %ebp
	movl %esp,%ebp
	movl 4660,%eax
	movl %ebp,%esp
	popl %ebp
	ret
.Lfe1:
	.size	 foo,.Lfe1-foo
	.ident	"GCC: (GNU) egcs-2.91.57 19980901 (egcs-1.1 release)"

--ADZbWkCsHQ7r3kzd--

--YD3LsXFS42OYHhNZ
Content-Type: application/pgp-signature

-----BEGIN PGP SIGNATURE-----
Version: 2.6.3i

iQBFAwUBN2OWvzyNAitYx2a1AQHgRgGA5fYIevSaZao25ZMudtaN+PDAbLIze3rn
aChV9rUthqm8lOp2nsD313AiGxpJwhr/
=eKxI
-----END PGP SIGNATURE-----

--YD3LsXFS42OYHhNZ--
----------------------------------------------------------------------

The `micalg' and `protocol' parameters in the primary `Content-Type'
header are vital, but it seems as if they aren't passed to functions
listed in `gnus-mime-multipart-functions'.  In addition, everything
between the first and second `--YD3LsXFS42OYHhNZ' boundary is covered
by the PGP signature, including the MIME headers and boundaries, which
get lost during the translation from raw message text to MIME handles.

[NOTE (January 2000): `mml-generate-multipart-alist' solves this.]

Finally, in the `multipart/encrypted' case, as shown below, the encrypted
part contains data which has to be put through the MIME translation
process as well.  This has not happend during the first translation,
because obviously, the data contained in the part wasn't readable then.

----------------------------------------------------------------------
Date: Sun, 13 Jun 1999 12:17:23 +0200
From: mutt@cygnus.stuttgart.netsurf.de
To: Florian Weimer <fw@deneb.cygnus.stuttgart.netsurf.de>
Subject: test
X-Gnus-Mail-Source: directory:~/Mail/INCOMING/
Message-ID: <19990613121723.A2917@deneb.cygnus.stuttgart.netsurf.de>
Mime-Version: 1.0
Content-Type: multipart/encrypted; boundary=Kj7319i9nmIyA2yE;
	protocol="application/pgp-encrypted"
X-Mailer: Mutt 0.95.1i
Lines: 24
Xref: deneb.cygnus.stuttgart.netsurf.de misc:177


--Kj7319i9nmIyA2yE
Content-Type: application/pgp-encrypted

Version: 1

--Kj7319i9nmIyA2yE
Content-Type: application/octet-stream

-----BEGIN PGP MESSAGE-----
Version: 2.6.3i

hIwDbSMVHQQS92EBBACFUjqnO8eDDinYfo183PYgd7GQInt3BHhZCnRP807uGwra
F2zAc4PBgGhBKYnU/nZiOZvqYVh+CA7JqjTzku0sM12RLg2R6bg2nB+V+CzOkK8R
5TQczKGI/Tyhs/ktej0pbMxR5rt5FwvDuB8hcCNKPPTfaeu/4RQsfnlaxsJXm4Q8
AzyNAitYx2a1AQGAug9oiCsl6+omafWxqlJBohEjXJCKjAASE9pfBHNqk6vhUMx3
qbLv6ai7AXOa9voApgAAAFwh2dnqGHsvY9WxPhIGgwkE9/CdjCX8p+jIG50F05Qe
HUyZLv61ibRCm8EYEO2SBUO0brS2u7wtJuOhtGO03SLiDuPuZh2gaqIkpB7xwDxv
T3BUKwC4W9ZIVybX7A==
=oXnI
-----END PGP MESSAGE-----

--Kj7319i9nmIyA2yE--
----------------------------------------------------------------------

The encrypted data looks like this (but it might contain attachments
and other MIME mess as well):

----------------------------------------------------------------------
Content-Type: text/plain; charset=us-ascii

Just another test
----------------------------------------------------------------------


IMHO, an appropiate hook into `mm-dissect-buffer' would allow
to do all these things in a relatively straightforward manner.
`gnus-mime-multipart-functions' could be used to put information gathered
during the MIME translation process (signer, valid/invalid signature,
beginning/end of signed data) into the *Article* buffer in a nicely
formatted form.

[...]