Please see the image here, made in Illustrator by faking it, but I'm wondering if it's possible to make this by setting a line as the point at which fills reach, without making closed shapes.
Or any other way to do this:
Revision FX' RE:Fill can be used with masks to achieve a somewhat similar effect, but otherwise you need to adapt your thinking. You have things backwards. There's a million ways to apply gradients and strokes/ outlines to closed areas, but determining open boundaries is an exercise in itself from a mathematical point of view, so even if AE had such a feature/ effect, it would not necessarily always work in such situations...
Mylenium
So the answer is "NO, boundaries cannot be set by lines relative to a direction..You have to fake it, by making closed shapes and then masking them off to avoid gradient fill spill occurring where you don't want it."
Thank you.