You may have seen the demo of user-defined functions from the Cloud Platform Live event in march. We're currently looking for trusted testers for this functionality. Essentially, you can write a javascript function that can read and emit rows of data. While this is still an early preview of this functionality, it is pretty robust, using the V8 Javascript execution engine.
If you're interested, please contact your cloud support representative and ask about joining the UDF trusted-tester program. If you don't get a response, please feel free to e-mail me at tigani at google, and I'll route to the right place. (we are limiting the number of trusted testers that we sign up, at least at first, however).