@LVS8Barrs Developer,
I'm seeing now that my original question may have been confusing -- what I'm looking for is not to have the ability to trigger a HomeKit Scene via a Shortcut automation, but (kind of) the other way around. I want to have a HomeKit trigger (such as motion detected) be able to trigger the Siri Intent of a 3rd party app.
The 'Intruder' scene that I mention above (flashing a light) can be achieved in the Shortcuts app, but this requires the user to program it themselves, which is obviously not the majority of users. I have an app in the AppStore that can be manipulate the HomeKit lights in the home for communication purposes (flash the kids lights with "Hey Siri, Dinner Time", etc), and my #1 most requested feature is to trigger the shortcut on motion detection and button presses, but today (and in iOS 14, currently), it can't be done.
The other aspect with my app is for accessibility purposes. Light manipulation can be used by those with a hearing disability, and a large percentage of this audience is older - I would like to allow this integration to be made as simply as possible, but today, it's not possible. My father is in this audience, and doesn't even know what the Shortcuts app is, much less have the patience to figure out how to flash a light with it.