I think this is part of a lighter weight Kubernetes alternative.

Lighter than the existing light weight kubernetes alternatives i.e. k3s :)

Or, hear me out: Kubernetes alternatives that don't involve any parts of Kubernetes at all :)

You still need some warm-up routine to run for the newly online server before the hand-off occurs. I'm not a k8s expert, but the above described events can be easily handled by a bash or fab script.

This is a health/readiness probe in k8s. It's already solved quite solidly.

What events do you mean? If the app needs a warm up, then it can use its readiness probe to ask for some delay until it gets request routed to it.

GET requests to pages that fill caches or those that make apache start up more than n processes.

I think the parent project, Kamal, positions itself as a simpler alternative to K8s when deploying web apps. They have a question on this on their website: https://kamal-deploy.org

"Why not just run Capistrano, Kubernetes or Docker Swarm?

...

Docker Swarm is much simpler than Kubernetes, but it’s still built on the same declarative model that uses state reconciliation. Kamal is intentionally designed around imperative commands, like Capistrano.

Ultimately, there are a myriad of ways to deploy web apps, but this is the toolkit we’ve used at 37signals to bring HEY and all our other formerly cloud-hosted applications home to our own hardware."