@JR, that also works well, thanks.

Perfect, thank you! The second option worked well. Just the graphical tweak I was looking for.

Fair point about the perfect overlap @Spacedman, I just assumed there should be a graphical tweak that would allow display of both lines. The shapeburst fill is close, although the display is no longer as clean. I might have to go with insetting the polygons a little. It is only for graphical display, so messing up the geometry is not too much of a problem. Thanks.

The raster functions have now been incorporated into the terra package - rdrr.io/cran/terra/man/aggregate.html, but otherwise this answer still works.

I had the same problem, but as of QGIS 3, there is a tick box in the GUI for 'Do not use the default validity mask for the input band'.

For anyone looking for how to do this on Mac, make sure to scroll down to @JDahlgreen's answer which has some useful tips, especially about changing the download extension from .txt to .run