Hi Folks, I was able to get it working!
I was pretty close to making a native plugin for Construct 2. It recognized my app started and all that, but couldn't figure out how to have the Cast API listener always running.
So I reverted to my nasty hack.
After exporting the game, I simply edit index.html and add my external javascript API I wrote for Chromecast. Works like a friggin charm. I make heavy use of c2_callFunction("myFucntion", [1,2,3]);
The more I use Construct 2, the more I absolutely fall in love with this. I'm going to make SO MANY APPS. You guys rock.