Emily,
Nice example, but I agree - very challenging to evaluate!
To be specific, are you interested in the health effects on the people incarcerated because of 3 strikes or on others in their families or communities or people at risk of incarceration? It's important to be specific because the feasible research approaches may differ between these populations.
For estimating the effects of 3-strikes on all-cause mortality of people incarcerated due to the law, maybe, and I don't know the details of the three-strikes laws well enough, we could find arbitrary features of people's crimes to compare. For example, you could compare people whose first crime was just above or below the age threshold to be recorded, or people whose first crime was not prosecuted because of a non-aggressive DA or not convicted d/t the judge presiding.
Maria