Question: How do I let a few specified non-subscriber email addresses post to an otherwise closed GNU Mailman mailing list?

Answer: 
I finally figured out how to do this!

Go to the list's Privacy Options, click on Sender Filters, then add the emails to the option called `accept_these_nonmembers`.
I'm not familiar with Mailman itself, but an external workaround would be to give them a dummy address to send to that will auto-forward to the list on their behalf. The dummy account can be a 'Subscriber", and restrict incoming mail to only accept from your desired contributors.