There is no documentation on Stagefright. I haven't tried implementing a new stream type yet, but it does look pretty straightforward:
1. Create a FLAC stream extractor. See MP3Extractor.cpp or AMRExtractor.cpp for examples of audio stream extractors. It needs to be able to handle things like sniff a file to recognize FLAC content, seek in the stream, resync, etc. Add the new extractor to the list of stream extractors registered in DataSource.cpp.
2. Create a FLAC decoder node. See MP3Decoder.cpp or AMRNBDecoder.cpp for examples. Add the new codec to various places in OMXCodec.cpp so that the player engine can find it. Add the new mime type to the list of supported formats.
3. Add the FLAC container format to the MediaScanner (3 place if I remember correctly). This may already in Kenny's original FLAC submission.
4. Add FLAC to the unit tests in the media test framework.
This took all of about 5 minutes of spelunking in the code. The trick is just to use the existing audio formats as a guide.