This is a great question and thanks for all the details!
When you’re configuring the Endpoint Reply, you can select a stream source:
Here, you’ll want to use a custom topic. Like /event/alerts:
You can even go further, and set up some pretty flexible experience user specific alerting with /events/{{experience.user.id}}/alerts.
Then, in another workflow, you can use the Timer Node, the Event: Get Node, and the MQTT Node to publish messages to your custom topic. Every time you publish, it will send a message to your SSE stream.
Let me know if you have any questions. Happy to explain further.