Your issue here is less about study design than it is about research ethics, as pointed out in the comments. It is not correct to simply state that we can suspend ethics, because the responses would be meaningless. It's like asking, "If Australia played New Zealand in women's netball on the surface of the planet Venus, who do you think would win? I know this is impossible, but let's say this could work." EDIT: Sorry, that's a poor analogy because there was no clear ethical conflict.
However, there is a way to design an experiment that respects current human research ethics principles. We do this all the time in public health.
Let's say you wish to see if smoking is causally linked to alcohol consumption.
Option 1. Study population - people who smoke. Randomise participants to a program (device, chemical, activity) designed to stop smoking versus some comparator such as no program. Then, measure rates of alcohol consumption.
This won't help you if causation induces in participants a long-lasting or permanent change that influences the outcome. If smoking does cause alcohol dependence, say, then the idea of stopping smoking can't help those that are already dependent on alcohol. In this case, you can use Option 2.
Option 2. Study population - people who do not smoke. Randomise particpants to a program (device, chemical, activity) that prevents the uptake of smoking. Then measure rates of alcohol uptake.
As far as I know, these are the only two broad experimental options open to you that respects human ethics concerns.