Something like this though could help to make labor a more realistic market. We can comparison shop almost any product via walmart or amazon, etc. Why shouldn't it be that way with employees?
What I need:
* A wide range of employers who provide a shortlist of what they consider to be the factors that define the top 1%, 20%, & 50% in any given skill set plus a respective list of appropriate salary levels for said skill set.
* A list of employees who fit into each criteria and their average, current salary.
You would then need to source that data for every geographic region.
TL;DR: It's theoretically possible, it simply requires a lot of grunt work and co-operation & honesty from employers & employees.