I have a project with Appengine part and Google Containers cluster. Appengine app needs to make http calls to a Service deployed to Google Containers.
I know that I
On standard App Engine, the only way to access your instances is through the appspot.com frontend. On Managed VMs you can also get the external IP address for a single instance from the Developers Console (which is still ephemeral) and access it that way but that's as close as you can get and defeats scaling.
One reason why this kind of feature doesn't exist for App Engine is because instances can spawn in any zone within the same region.