I was hoping someone with more knowledge about this subject would hop in here and share what they know. I have messed with Google Assistant once, but that's about the extent of my experience.
Firstly, I don't think there's any way to get any kind of voice-activated app to work from the lockscreen.
You can try Voice Access. It uses accessibility services to work, so it might have more privileges than a regular app. There's a way to activate it with a floating button on your screen.
There's was a bit more discussion in another thread. Maybe you can browse through it and maybe glean some more useful information to get the your desired result?