I usually ask the question in an open way, but guide the candidate to "fast forward" past the (for us) boring parts, and focus on the interesting parts we are looking for along the way.
If your OS covers the concepts we are looking for and the answers seem competent, sure, let's give it a try, but don't be surprised if we veer off of that. If the OS is too simple to cover everything (not unlikely for actually existing toy OS projects, I have one myself), we'd want to branch out to those concepts no matter whether you included them in your hypothetical OS or not. And if your made up OS is just too different, it might not apply either.
In other words, ask yourself why this is part of the interview, and that should answer your question: Because you are going to work on a particular OS with particular characteristics.
If you "rote memorized" both the high level view and the raw details so well that you can freely think and talk about it during the interview, I'd say you'd have to have had some experience as well.
However most people are unlikely to have done that.
Instead if someone can provide a good answer they are demonstrating a deep understanding.
