How to fully automate from build to .ipa file in xcode 4
Sure, the script I have been using in my continuous integration environment is available at https://gist.github.com/949831 That might do more than you need but should be able to serve as a base for whatever steps you want to include in your build process.
As you noted it would be hard to predict the derived data path Xcode will use but it is not too hard to located it in the build output as part of the build script.