Thanks for your feedback. I have a solution.

The problem I was seeing in my tests occurred when trying to send a message before the C node was listening.

    Elixir                        C node        
-------------      ------------------------------------
  Port.open()  --->   OS process starts running C node
                                    |
  Send message ---> (X) not ready, Epmd connection fails
                                    |
                     (ei_listen, ei_publish, ei_accept)
  Send message --->    Epmd connection to C node (OK)

Comments from @benwilson512 and @ityonemo made me think, “What if the C node initiated the connection back to the Elixir node, instead of the other way around?” And maybe that’s how they were thinking it worked (or should have worked) in the first place. The initial code was just from sample C node code found on online, where the C node published itself to Epmd and listened, waiting for Elixir nodes to connect, as diagrammed above.

So I changed the C node to initiate the node connection, rather than just waiting for a connection:

    Elixir                     C node
--------------      --------------------------------
Port.open()    ---> OS process starts running C node
                                 |
connected (OK) <---        ei_connect()

And after I remembered that C nodes are hidden, and used :net_kernel.monitor_nodes(true, node_type: :hidden), then I did get a :nodeup message!

This also simplified the code on the C node side, I can just call ei_connect, and don’t have to use ei_listen, ei_publish and ei_accept.