iOS app store submission - including a screenshot that isn't a screenshot of the app
There's always a risk of rejection of an App store submission for not strictly following the guidelines.  Best to check what other popular apps display (some have variations that are not exactly screen shots, but photoshopped artwork similar to such) to see what they get away with.  But realize that Apple's previous approval of these apps does not set any precedent for continuing to do so.

You could try putting a help screen in your app, and display a smaller image of the SMS messages window within your own help screen.  They you really would be submitting a real screenshot of just your app.