But completely ignore Fitt's Law. Usually I can speed my mouse to the right of the screen, click, drag. Here I speed to the right of the screen, maybe try clicking and dragging, fail, move in 30 pixels, overshoot, move back and finally can scroll.*

The problem with allowing the customisation of scrollbars is that most people are going to make them worse.

* Personally I use the scroll wheel, but I know a lot of people who don't.

Very true! I'd probably make them flush with the right edge and slightly wider, based on that.

I'll bet he could widen the target area while still retaining the original design.

To what gain? Custom scrollbars are unnecessary on this page. It makes sense on a scrollable div with lots of content around it; big fat default scrollbars are distracting in that scenario. But this page is a document. It breaks the expectations of the user (what is this thin line on the right of the screen and why can't I see the rest of the document?).