Economic mobility is usually determined by things like IQ, EQ and other marketable skills. So I don’t really know if your proposal is the right way to measure it. But such data would at least give some insight.

In the USA, most research I have seen says they have low economic mobility, because the rich have access to the best schools, etc.

But still, it’s not zero. Both JD Vance and AOC are examples of economic mobility.

One of them still fights (or appears to fight) for the class they came from, the other is successfully recruited to serve the interests of the ruling class.

Were they born in 1908 (and ignoring race and gender for the moment), then probably both of them would have been leaders for the working class.