I'd like to do some R&D pushing the boundaries of iOS devices. Is there a how-to guide or private API documentation?
Is there a Stack Exchange site for this?
There is a Stack Exchange proposal but it's lacking support:
[...]for iOS developers who require help and advice when using undocumented iOS APIs. Developers who discover private iOS APIs and need a place to document them.
http://area51.stackexchange.com/proposals/18154/ios-private-apis