Hi Dave,
Yes, it is possible to create a DNN page for a specific category.
After you add the BuyNow module to the page, when adding your category specific products there is the "Share products
between catalogs" feature on the Product Setup page that can be enabled/disabled to share products between the BuyNow modules. You will will want this feature to remain disabled.
When adding your Category in the Manage Catagories screen in the Admin menu, you also have the ability to enable/disable the "Share" feature which controls if your Categories are shared between your BuyNow modules. Leaving the "Share" checkbox will keep your Category isolated to that BuyNow module.
I also have an example set up on our www16 test portal. If you would like access please email me a kevin@smith-consulting.com to receive login credentials.
Thanks