It should be pretty simple to close the gap.
It isn't really. You should look at some of the triangulation routines in the archives. Basically, you would need to create a list of the vertices which lie on the intersection plane, in clockwise order IIRC and pass them to the triangulate function.
Getting them in the correct order is probably the most difficult part. Thinking about it, you could find the center of all the vertices which sit on the plane and store each vertex angle to the center point. Then sort this list based on it's angle. This would probably only work with simple convex shapes.
Stevie