I had the answer back when Sony started howling over their treatment from Apple.
If Apple is actually going to enforce the rule that if you want to access titles purchased outside of the app that they must be available for purchase inside the app, all you need to do is add one little internal purchase screen that just requires you to type in the full ISBN number of the book you'd like to purchase.
Given the choice between doing that or browsing via the website, I think the website would win.