I got an email from Google about the new Google Home app, and now I can see the “When a device does something” option.
Oddly, they removed “When an alarm is dismissed”. I figured they had added it to the new option, but nope. Unless I just can’t find it, it’s just gone. Not like I was using it anyway, but still…classic Google. 
Now that I can see it, there’s a relatively simple workaround for passing a command to Google Assistant. They’ve added the ability to trigger a routine based on a Google/Nest device’s volume being set between a range. So:
- Create a routine that triggers on a Cast device’s volume being set to a specific value. Let’s say 8% (it’ll be better to use a low volume than a high one).
- You can set the volume range in the trigger to be exact by making both the minimum and maximum 8%.
- Use the Chromecast binding to send a command to that device that sets the volume to 8%.
- Change the volume back to whatever your normal default is, either through an openHAB command or in the GA routine you’ve just triggered.
I tested this and Google fired the routine in about 5-10 seconds.
If you have a Chromecast, you can also trigger on media changes or on/off state. But for now, I think it makes more sense to use the volume since you can isolate a value that you’ll never ever use.
The reason I say “for now” is that this is only going to work until Google decides it’s silly to start routines based on volume changes and removes the trigger. You know it and I know it.
If you have an old Chromecast that you’re not using, you could use it purely as a bridge for sending commands to Google Assistant. Then you’d have 100 triggers and wouldn’t need to worry about the volume. For now.