Is there a way to access application data through Google Actions?

I'm just getting started with Google Actions SDK. I want to write an action that accesses an API which requires a private key. I could just use my private key as a stored secret on my fulfillment server, but I'd rather get the user's own private key, as every user with an account is assigned one. The Node.js Google Actions SDK can't talk to the phone, but I'm wondering if there's a workaround if a custom app was also installed on the phone? Could I fulfill the Google Action with a deep link into this app? (android deep links) It would basically be an async call to the app, but there would have to be a way to callback to the assistant from the app.

If that deep link is not possible then there would have to be a way to trigger the Google Assistant to broadcast a custom intent, which my app could handle and potentially callback (still only potentially, not sure if possible). This looks unlikely, here is the HTTPS response accepted by the Google Assistant. This custom app would serve one purpose: fetch the private key from a local db or dir.

It doesn't seem unreasonable that an action would at least be able to open an app on the user's phone. Maybe that's all just wishful thinking.