Support for a single app engine route (one route of one App Engine service in one GCP Project)?

Is it possible to get esp-v2 working with a single route in an app engine application? I thought perhaps this could be done by using a [Serveless NEG](https://cloud.google.com/load-balancing/docs/negs/serverless-neg-concepts) and the IAP attached to that, but I get inexplicable errors with every variation of that strategy I've tried.

With or without IAP enabled, when I connect to the cloud endpoint, I get the following error:
```
{
"message": "upstream connect error or disconnect/reset before headers. reset reason: connection failure",
"code": 503
}
```

In the logs I see this, in case that helps:
```
D0917 04:32:00.835 28 envoy] [28][connection][external/envoy/source/common/network/connection_impl.cc:753]
[C23] connecting to 127.0.0.1:8082
...
upstream reset: reset reason connection failure
```

Any advice or pointers to docs would be welcome, thanks!