To answer my own question, it seems this is not possible.
There are hooks in the Cloud Endpoints code to change the cache-control, but they are stubs, not yet implemented: https://code.google.com/p/googleappengine/issues/detail?id=9042
ProtoRPC does not yet support it either: https://groups.google.com/forum/#!topic/google-appengine-python/l43R6w_3mpM