Is it possible to query App Engine’s Datastore from outside the cloud, i.e. a client application?
I could possibly write an app to be housed within AppStore and query the Datastore returning XML-formatted data; I want to know, however, if there are any Datastore endpoints which would allow me to do it directly.
Also, in case it is possible, am I able to do so via SSL?
Yes. The remote_api library supports exactly this use-case. If you’re using Java, there’s a Java remote_api handler available, and the client will be available at some point in the future.
You can use this over SSL in the same way as any other handler.