You can create a WinRT component DLL in managed code and include that in an otherwise native app.
The automated validation check is done via the Windows App Cert Kit (included in with the Developer Preview Tools), so you can run it on your project locally. See Ted Dworkin's talk "Introducing the Windows Store" at
http://channel9.msdn.com/events/BUILD/BUILD2011/APP-121T for information on the submission and validation process.