It seems individuals are able to adjust their cognitive aptitude to some degree through nurture, which is controlled by various elements such as environment and conditioning. However, they're stuck with their nature - heredity. Is it possible to measure and compare one's capacities to occupational cognitive demands?
According to the United States Armed Forces, it is possible. They do something similar to what you propose with the Armed Services Vocational Aptitude Battery. However, according to the section "Test Validity" of the linked Wikipedia article, there is some debate about whether it only measures literacy or IQ.
A lot of time and money has been invested in finding a good way to measure one's natural "aptitude" at mental tasks, and as far as I'm aware nothing has yet come close to IQ. At least not in terms of predicting an individual's ability to perform mentally challenging tasks, to learn information quickly, etc.
If you have an IQ of 80, you aren't going to be able to thrive in a job role that requires rapid problem solving, abstract thinking, complex pattern recognition, and multitasking. That's not saying you can't be extremely successful at something else, but IQ is a good predictor of "occupational aptitude" in many areas.