Unfortunately, this would require a redesign of the entire notebook because the leather piece would need to be sewn into the back cover. The good thing about the pen band is that you don’t have to buy a new notebook in order to use it. Still, I’d prefer the option of a William Hannah notebook with a snap closure or a closure that integrates with the current metal button.
That said, if you haven’t taken a look at William Hannah notebooks, you’re really missing out. They are absolutely exquisite and the paper refill options are so useful (see here for a review of the wonderful Intentions Pages refill). The notebooks come in A5, A6, and pocket size.
You can purchase a pen band for the A5 and A6 notebooks. Both cost £29.00 (about $38 USD).
(I purchased the William Hannah Pen Band with my own funds.)