I use a generic version of this.

My .gitconfig has:

  [pretty]
  co-authored-by = Co-authored-by: %an <%ae>
  [alias]
  co-authored-by = log -1 --pretty=co-authored-by --regexp-ignore-case --author

Now `git co-authored-by Tom` generates a Co-authored-by: trailer for the last person named Tom who committed to the repo. Typically I'd just do `:r !git co-authored-by Name` in vim (mapped to \gc to save typing).