Hey Nick, sorry for the delay getting back to you.
PackageManager is a little atypical in that it's mostly abstract, so rather than shadowing it, we've implemented it as RobolectricPackageManager. You should be able to implement most of what you want there. RobolectricConfig.parseAndroidManifest is where the AndroidManifest.xml is parsed.
We'd love to get a pull request when you get this tested and working!
Thanks!
--X [typos courtesy of my iPhone]