It seems that the stream endpoint in the rest API is recommended for testing only and will stall if the throughput is high and Google App Engine doesn’t seem to support HTTP streaming.
We have not yet tried running DataSift on the Google App Engine. We know that it will not support web sockets, but the Java classes that suport HTTP Streaming do exist in the App Engine, so it should be possible.
Although using the REST API would work, you would be likely to lose a few Tweets - particularly if you are running a high throughput stream.
This is definitely an interesting concept, and is something we will look into very soon.