There is a much more recent paper called Ultrasound based measurements of sugar and ethanol concentrations in hydroalcoholic solutions.
Was there a reason you rejected ultrasound as an option? Seems that a buoyancy based method would introduce contamination concerns (and extra cleaning).
@Elco any updates on this? What would be the go to solution nowadays with BrewPi?
I did come across https://tilthydrometer.com/products/copy-of-tilt-floating-wireless-hydrometer-and-thermometer-for-brewing which looks like a good option.