I remember this story! It was in my mind when I recently wrote these two notebooks: https://observablehq.com/@mcmcclur/trivariate-choropleth-for-racial-composition-in-the-us and https://observablehq.com/@mcmcclur/racial-distribution-in-nc

Very dope! I really enjoy trivariate scales for projects like these. I'll be sure to try your approach when I get a chance.