Over on https://bugs.debian.org/840398 , Dominik George wrote:
When using GPG keys for SSH through gpg-agent, it would be great if,
instead of "(none)", the primary UID of the main key of the
authentication key used could be used as comment.
I'm aware that this request isn't implementable exactly as requested. For one
thing, some authentication-capable subkeys could be attached to multiple primary
keys, each of which has a different primary User ID. So that corner case makes
the generic solution imperfect.
It looks like these values are stored in the ~/.gnupg/private-keys-v1.d/ in the
"comment" element of the S-key, which is populated when keys arrive in the agent
via ssh-add directly. While users could explicitly add such a comment to
pre-existing private keys, it would be nice to automate this process for
authentication-capable subkeys that the agent already stores and aren't
otherwise bound to multiple primary keys.