How to invoke navigation on Google maps using Siri? At the risk of getting a big NO, 

Is there a way to use SIRI to invoke navigation on Google maps as opposed to the default Apple maps? 

I have tried "Navigate to Chicago using Google maps", but Siri just treated "Chicago using Google maps" as the location itself.
Yes you can
Simply append &quot;via transit&quot; to your Siri query.
Ie: Directions to Miami Beach, Florida via Transit.
Siri will determine your target location, and will give you a choice of transit apps. Tap Google Maps and you'll be on your way.
It appears these words also work:

by transit
using transit
via transit