How do you create a direct Google Hangouts contact on the home screen?
This is now available as an option in Hangouts 7 which started rolling out in February 2016.  

To create a shortcut...  


Open a Hangouts conversation.
Tap the 3-dot overflow menu in the top-right.
Tap "Save to Home Screen"


A conversation icon with will be placed on the home screen ready for quick access.  It works great!