Are there any Cocoa constants for the various resource suffixes, currently @2x, ~ipad and -568h?
No there isn't.
You can see some documentation here (and in some other places) in Apple doc about these strings to name your resources, but there is no Cocoa constants for them.
Just to be sure, I tried to grep the strings in the SDK directory (/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS6.0.sdk/System/Library) and those strings were only found in the framework binaries, not in any header file of the SDK.