There are two ways to do this. The first is using the “cast” method, where a middleman server casts an MP3 file to you google home. This would allow you to send a message to a specific Google Home (or home mini) speaker).
And the second is using assistant relay. I just got this working and it’s a bit more complicated. This uses the google home broadcast feature. It sends a message to all google home’s on your account but it allows any music you are casting to the device to continue after the interruption and also allows for some fun presets (like the rooster for the wakeup broadcast).
You easiest thing to do then is to use WebCore to send the message either via the speaker using “Speak” action or in the case of assistant relay, using a POST message in JSON format.