All your reasons why IJD is not suited to a SE Q&A are reasons I think it would be perfect.
Velocity of change
iOS does not change that quickly. And with Apple current release schedules private API will probably change about as often as public ones.
Irrelevant Questions
This site will be populated by iDevice hackers, they will want to hack a myriad of hardware and software. Question and Answers will almost certainly have to be tagged with appropriate iOS version and hardware models. But I feel this is a SE strength not a weakness.
Scope
- 5 iOS releases
- 5 iPhone models
- 4 iPod touch models
- 2 iPad models
- Apple TV
That is a lot of software and hardware, not all hacks work on all of them, this would be a great place to document that. I dis agree that information would be mutable, APIs in 4.3.2 on a iPhone 3G will not change, and just because it's not the latest does not make it irrelevant.
Conclusion
I agree this site will be smaller that others and it will have elements more akin to a wiki. But not every sites on the network should be a carbon copy of Stackoverflow.com.
There is nothing else on the internet that would come even close to this.