Peter,
It looks like Mojave now has an SDK for Siri: Sirikit
https://developer.apple.com/documentation/sirikit
For the time being I am creating some KM Macros that interact with "Type to Siri" to turn HomeKit devices on / off. It works - but obviously it's UI and who knows how reliable this will be. Some integrated Siri functionality in KM would . be . amazing! ... v9????? 