OK.
I've taken a look at the way 1.6.2 does things, and there isnt a way to use the Boards Manager for anything other than Arduino LLC boards
I strongly suspect this has something to do with the ongoing internal battle
I've had a go at updating the package data file, but the IDE reloads it every time the popup is opened, and overwrites whatever new data I put in the file.
What is really needed is a way to add external repo's to the board manager, but if the reason for it is truely to get back at Arduino Srl, then I doubt they will take kindly to any additions to the IDE to add external boards that Arduino LLC don't manage
e.g. they won't want the Zero Pro being added I suspect.
However in terms of the Arduino community and what the IDE is supposed to achieve, adding additional e.g. untrusted sources, like you can do for Linux distro's seems the logical method
Anyway, I've cloned the whole IDE and I will work on a pull request to do just that
In the mean time.
I could modify the repo to work with 1.6.2 by changing the compilor path
Or if worst comes to worst I could repackage the compilor as part of the repo, as its possible to include external zips into a "Release" package
Its kind of hard to know what to do for the best at the moment
Any ideas ??