Hrm... A while ago, I worked on a project that is somewhat like what you described in that it's a cart where you have the full ability to use any CMS you want

Hrm... A while ago, I worked on a project that is somewhat like what you described in that it's a cart where you have the full ability to use any CMS you want (or none at all) for product listing pages. There wasn't an API, though—most of the cart-specific functionality was done in JS. It's just a different approach to solve your problem. I never finished the project, but it's really close to being ready for launch. I'm interested in discussing it with you further! Shoot me an email (it's in my profile).