I figured this out. In case anyone else ever gets into a similar predicament, here's
how I did it. It seems this could be much easier if the change-passphrase option took an
optional --user argument so that an admin could affect someone else in one step. Instead,
I had to do the following.
LUSER=me # who forgot his passphrase
HELPER=them # who can be trusted as an admin and does remember their passphrase
KEY=f24 # the key for which $LUSER forgot his passphrase
# Make the other (trusted) user an admin.
sigul modify-user --admin yes $HELPER
# Make them a key administrator of that key.
sigul modify-key-user --key-admin yes $HELPER $KEY
# Now have them reset my passphrase by revoking it and then granting it,
# which will allow them to set an initial passphrase to something that's
sigul revoke-key-access $KEY $LUSER
sigul grant-key-access $KEY $LUSER