App Engine OAuth Callback to a Custom Protocol

I'm working on connecting an Android application to an App Engine server with OAuth. The common way of doing this is specifying a custom, non-existant protocol for the callback URL (e.g., myapp://) and setting the Android Manifest to filter intents with data pointing to that protocol.

My issue is, App Engine seems to error on any callback URL with a protocol other than HTTP or HTTPS with the error "Invalid Callback URL". Am I being an idiot, or does App Engine n开发者_Go百科ot allow this? How can I work around it?

From the feedback I've gotten, looks like this is impossible. Recommended route is the Accounts API.