Patrick, I'm sorry but optical density and mass or "density" of a material in units of weight have little relationshiip when it comes to particulate matter such as silver in gelatin.
There is a relationship between the mass of organic dyestuffs and weight per unit area, but it is usually only linear with mass if the dyestuff is cast, not dispersed as in particles and it does not work on reflection support, only by transmission density.
This relationship is unique to each organic dyestuff and is called extinction coefficient. Some of these values range up to the hundreds of thousands of units / mole of dye.
Silver metal or indeed any colloidal metal has no such thing as an extinction coefficient and can have none as they do not transmit any light. Colloidal metals allow light to pass through interstitial gaps and while doing so the light bounces around in there with some being absorbed due to the nature of the particles, and hence the color of the colloid. The size and shape of the colloid affects the frequencies absorbed, and therefore silver colloids can be made in practically any color you wish by adjusting the size of the particles. This, of course also affects the absorbance per unit of mass.
You might think of dyes as being like the atmosphere absorbing light, while silver is a haze in the atmosphere and scatters the light changing its color by the scattering power. That is a very rough analogy.
The dyestuffs do transmit light. That is different.
Early workers in this field were incorrect to assume a linear relationship between the mass of silver and its density under all conditions. That does not obviate the work of H&D, because there is an exact relationship between each unique silver image vs log Exposure for each development condition. Therefore, the work of H&D stands, but is merely a reference for the exact test conditions applied. (developer, time, temp, emulsion and instrument for example) And, as a test condition it can be repeated with exactitude if the calibration of the test is maintained. Therefore, ISO values and contrast values can be measured and obtained around the world, but only if the standards are adhered to.
On average, I use 60% myself going down the drain (so to speak). And for paper, I assume 100 mg/dm^2 and for film I assume B&W and color to be 300 mg/dm^2 as reasonable starting approximations.
PE