Thanks to liberal debugging via Firefox and the firebug plug-in, I have working two-way communication from the browser to the app engine via the channel functionality. I have almost no idea how it works… but it does!
Mostly just for test purposes, I made the same map (in HTML5) click-able and it now dynamically updates (ajax style I guess?) the terrain type at the bottom of the page. Seems a bit slow but at this point I have not done any kind of optimization or testing so no surprises there.