I think we all need to step back and check how measuring solubles lead and concentrations would work in the real world. I see two possibilities:

1. Quality Control
a. Get the right dose, grind, shot temperature, pressure and volume for your espresso. By "right" I mean the one that you think tastes best.
b. Find the extraction numbers for this.
c. Make sure the shots you are making and/or selling conform.

2. Finding Alternatives
a. There is a hyperplane of constant solubles extraction and concentration in the "space" of extraction variables (dose, grind, shot time,etc). In other words, you can change dose, shot timing and volume in such a way that the extraction and concentration stay the same.
b. Now if taste is more directly correlated to these later figures, being able to tweak dose and shot times to either get to a target in solubles or while maintaining that target could be very useful.
c. The same can be done for brewed coffee. If both coarser grind at 4 minutes steep and finer grind at 3 minutes steep give you a 20% extraction, will they taste the same, or are there boundary and local effects we need to know about?
d. Many of Barry's and Nick's reservations presuppose that an exploration like this will produce surprises. But you can't do the exploration without having these shortcut instruments (trust me, I know, unless you want to compete with Jane Goodall in persistence, you do not want to bake pucks to get enough data to answer questions like these)