SocketStream.putback...?

def push_back(stream, data):
    stream_type = type(stream)

    class PushbackStream(stream_type):
        async def receive_some(self, max_bytes=None):
            if max_bytes and max_bytes < len(data):
                ret = data[:max_bytes]
                del data[:max_bytes]
                return ret
            self.__class__ = stream_type
            return data

    stream.__class__ = PushbackStream

Got the job done.