The new 3.2 and 4.2 have a dedicated button for calling the Google Assistant app. AFAIK it cannot be remapped, Google forbid this, however, I wonder if it can be made to call another app by changing the default "assistant" app. This can already be done in Android 8.1, I don't know if it's possible in Android 9. Any app which say it is an "assistant" can be made to be default and then called by long-pressing the home button e.g. Firefox!
So, the question is, does this work for the dedicated assistant button?
Let me know when you get your 3.2 or 4.2!