Record audio on Android with MediaPlayer as source?

On Android, you can record audio from the microphone using the MediaRecorder class: MediaRecorder.setAudioSource(MediaRecorder.AudioSource.MIC);

I want to record some parts of audio being played by a MediaPlayer instance instead (or some other audio playback device). Is that possible? How would I set a custom audio source?

To summarize; how do I stream audio into the MediaRecorder from an arbitrary audio source?