Thank you to Gerd V. Egldy (via @VinayDand) and @PzP for the diligent documentation on your methods. I don’t know how I overlooked this missing feature for so long. Now that I’ve had reason to apply hatched fills (yes, one of those picky CapSense sensors), this thread has been crucial.
That all said, I abandoned both the documented solutions! Trying to maintain control over edges and dimensions between vector export, rasterisation and re-vectorisation through 4 separate software tools just became too much. And the result is too difficult to tweak, due to the prolonged edit-preview cycle.
In the end I drew it by hand, all in KiCad. KiCad’s trace drawing behaviour is definitely not ideal for this, but actually helped surprisingly well. Being able to set a grid, copy and paste, still get track snapping, and work directly on the PCB allowed consistent, albeit arduous, progress.
See attached for the result. Like with painting, all the hard work is in the cutting in. Doing so with straight edges was probably the biggest limitation, but if you’re willing to have a bit of arbitrary aliasing in your “curves”, it’s fine. Zoom in to see the details.