H264 de/compression

Is there a way/library available to perform H.264 compression/decompression on the device? I want to process/modify a video stream, then compress it for delivery across the network, and the Native Capture APIs don't allow us to modify the video stream before compression, as far as I can tell.

On Windows Phone 8, you can use AudioVideoCaptureDevice.StartRecordingToSinkAsync to get h264 samples, which are encapsulated in an NAL. I don't know any library to manipulate them but it's a good start.