It's curious that in both cases you actually have to create a special app to enable any custom behavior from your phone. It would be nice if there were a bit more extensibility in these automated assistant/search/whatever thingies.
Yep this is kind of unfortunate; it's only because Google Now only resolves strings of the form "open {appname}" and doesn't provide a way (that I know of at least) to register arbitrary phrases.
I also wish Google would allow some kind of intent resolution based on, say, regex matching of the voice recognition result. I want to be able to register phrases like "OK Google, turn on the lights" or "OK Google, set the temperature to 25 degrees" and allow my app to handle them (and yes I do actually have all my lights, thermostats, etc. controllable from my phone)
It's curious that in both cases you actually have to create a special app to enable any custom behavior from your phone. It would be nice if there were a bit more extensibility in these automated assistant/search/whatever thingies.