Did a little research for reference to anyone finding this post in the future!
Here’s the code in Phoenix.Socket.Transport to support a {MFA} 3-tuple to initialize the session configuration:
phoenix/lib/phoenix/socket/transport.ex at main · phoenixframework/phoenix · GitHub
The original issue: Missing capability of loading runtime config for Phoenix.Endpoint.socket · Issue #3659 · phoenixframework/phoenix · GitHub
And the pull request adding the feature: Allow setting mfa for loading socket session config by kafaichoi · Pull Request #3668 · phoenixframework/phoenix · GitHub
