Great question. Just to make sure I have it right, the causal model for your example is:

Lung Disease <-- Coal Miner --> Smoking --> Lung Disease

And you want to quantify the causal effect of Smoking on Lung Disease when Coal Miner is unobservable.

In this case, you would not be able to compute the effect since Coal Miner is a parent of Smoking thus it is necessary for the calculation.