Is it wrong to use NSPersistentDocument with an alternative save format?

I’m thinking of creating an application that focuses on core data based on Core Data, but I want the saved file format to be my own proprietary package of XML, images and other stuff.

My plan is to use a subclass of NSPersistentDocument that serializes back and forth between my file format and Core Data when receiving calls like -writeToURL, -readFromURL, etc.

Is this an anti-pattern? I started experimenting with it and it looks like this will work, but I don't see much documentation / discussion about this approach, which gives me a pause.