An offline crack for Diablo 3 would be like an offline crack for WoW. Anyone who actually wasn't trying to throw a trojan on your computer would not call it an offline crack.
It would be a emulated server that you'd have to run on your own machine.
Just to reiterate, an offline crack for Diablo 3 is not possible. Running your own emulated server is very possible, but will be a crap experience just like trying to run an emulated server for WoW.