Field Overrides: Work in Progess

We just wanted to start a place for discussion of this feature as it evolves.  

Primarily for @cpennington @carsongee @pdpinch @davisagli 
Just a general question, but is the idea going to generally be you would have these overrides in the platform code already, or are we thinking of having a few in the core and then have them live in installable python packages or similar?  That answer may lead some other conversations like entry_points/discoverability and such for settings/configuration