re-issuing subkey binding signatures with alternate digests

[Daniel Kahn Gillmor]

Does anyone know of a way to coax GnuPG (1 or 2) to re-issue existing
subkey-binding signatures?  I'd like to refresh the signature over an
OpenPGP subkey to include different parameters.  What parameters?  One
example would be to add a usage flag.  Another (more relevant to my
current digest review) would be to change the digest algorithm used on
the subkey binding signature (e.g. to re-issue a subkey-binding
signature that was originally issued with MD5 to a more acceptable digest).

The only way i see to do it is to change the expiration date on the key,
which triggers the creation of a new subkey binding signature, but that
signature does not seem to respect arguments to --digest-algo or
--cert-digest-algo (it seems to currently re-issue the subkey binding
signature with SHA1 no matter what --digest-algo or --cert-digest-algo
is set to (i've tried with SHA512, SHA256, and MD5, all of which appear
to be silently ignored.

Is this intentional?  Am i doing something wrong?  The workflow i've
tested is:

 gpg --digest-algo SHA256 -cert-digest-algo SHA256 edit-key $KEYID
   key 1
   expire
   1y
   save

and i view the subkey binding signatures with:

 gpg --export --export-options export-minimal "0x${SUBKEYID}!" | \
   gpg --list-packets

	--dkg

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