Here is an interesting hardness test by the Canadian Youtuber "AvE" (His videos are quite interesting and entertaining.)
He's measuring hardness by bouncing a bearing ball in a clear tube and measuring the height of the bounce to determine the hardness.
https://www.youtube.com/watch?v=qUpape0XgVs
At the end of the video he identifies that such a device does already exist.
Still, it gets the brain thinking...
A more limited range of BHN measurement - maybe up to 30 BHN? I think it could be used for testing bars of alloy; not sure how practical on the base of individual boolits.
Optical sensors could be used, triggering as the bb passes, each spaced at the heights correlating with BHN steps, the sensors would be connected to a micro - maybe an arduino - and displayed on a screen.