While ideal for them to use their profile frames to identify themselves, I do believe it should be possible to do this if you're on a pro plan.
Could you test something for me? If you go to manage your members in the Slack settings (not Tightknit), do you see the ability to rename a person in the community? If you do, then we should have the API access to do the same.
Let me know and then we can make the ticket!
(I would test but our plans are a bit different)