Hi Raistlin,
It actually works exactly as Dragan & Rokas said. I just happened to be working on a similair script (meaning with a border) so I figured I'd try it out and I've attached the result.
Select the spline, outline it the exact value of the border (100 in my case) and delete the original line (or detach it, whatever fits your need best). If the border still crosses the rest on the inside, you just have to reverse the inside spline and there you go!