We're looking to enable this by default soon, no configuration required. That means that it will also work for sandboxes, assuming your sandbox isn't asleep :)
I'm afraid not. At least, I can't think of a way to circumvent the custom runtime setting.
I would love to test this new feature so I:
set the 2 custom settings as stated in the release notes
do I missing something in configuration? anyone got this working? plz let me know here!