The answer to your question about an include system is yes, and further, a JavaScript include system is the worst possible way you can create one. Unless your server does not support PHP, ASP, ColdFusion or SSI, I would strictly keep away from serving up critical information, such as site structure, using JavaScript. Stick with server technology. Don't rely on the visitor UA to interpret your site!
With that said, is there a specific reason you're targeting JavaScript, something in your environment that makes it the only option?