I'm really tempted to get one of these. Is it just me, or do they seem like the perfect platform for a home automation remote? Of course, a home automation remote wouldn't be too useful without a home automation back end, but I can dream.
The central really makes one miss the empeg community. The hardware is all there, but it just doesn't run hijack. Hijack's web server would add so much to the central, and I miss it every time I play with my parents central. I can't imagine why you'd want the sound to come out of the 770, but I can certainly see controlling what the central is playing through the 770's web browser.
To be a bit more on topic, the central protocol is pretty well documented, though you're looking in the wrong place. Look up the many replacement rio receiver servers, and you'll see what people have done. Then flip the spec over, and start reading it upside down, and you'll see what needs to be done.