To me, the whole point of Appengine is to abstract out system administration. GCE (like AWS) abstracts out the systems, but not the administration - you suddenly become responsible for all the scaling, fault-tolerance, and all the subsystems that Appengine manages for you.
I've tried Containers with Google App Engine Flexible Environment (in beta now) and liked that it's much more customizable than standard GAE. Basically, you just need any Docker container listening on port 8080. But deploy time took 10-20 minutes, so I'm still preferring Standard. Hopefully they'll fix that before GA release.
My request is for a tool which would provide this functionality in the form of code which exposes an App Engine like interface to the developer, except it's running in Container Engine. Given Container Engine has the required code to restart processes that fail and oodles of other automated goodies, this give the fault tolerance desired.
> We wouldn't use it.
Given the "tool" doesn't appear to exist IRL, that statement is illogical. You can't not use something that doesn't exist. And, if there really did exist a migration tool that provided additional operational abstraction inside the framework you were migrating to, you'd run the risk of being in conflict if you didn't consider using it.