As an additional evidence, one of our apps that included this file received "required screenshot is missing" iTunes Connect status upon update upload today, and it had all regular screenshots, but not iPhone5 ones.
Apple has a long history of doing `linked-on-or-after` checks for compatibility with various older paradigms or iOS versions. As such, I don't think throwing a screenshot into a directory will make the app actually run at full size. It might mollify iTunes Connect, but mollifying iTunes Connect is just one facet of shipping an app: the runtime is a completely different animal.
This is even a difficult condition to test, as installing an app in the 6.0 simulator without rebuilding it for the 6.0 simulator and thus passing the linked-on-or-after checks (and as such, with Xcode 4.5) is nigh-unto impossible, as the 6.0 simulator does not quite predate Xcode 4.5 as the beta versions could very well fall under the "or after" part of the link condition.
1.) I compiled my app with a 4-inch default image using XCode 4.4.1 with iOS 5.1 as the base SDK and ran the app in Xcode 4.4.1's iOS simulator.
2.) I copied the resulting i386 app folder from /Library/Application Support/iPhone Simulator/5.1/Applications/[app uuid] to /Library/Application Support/iPhone Simulator/6.0/Applications/
3.) I started XCode 4.5's iOS Simulator and ran my app from Spring Board.
The result? It did work. I was able to see my Xcode 4.4.1-compiled app in 4-inch display mode.
I can't say for sure if this will work on the iPhone 5 hardware, but I'd bet the answer is yes.
I guess with this finding, if you're not using any iOS 6.0 features, it may be better to continue to compile your app using Xcode 4.4.1 since that lets you more easily produce app with armv6 support.