Let's say the government proposed a rule tying Medicare type payments to your BMI (gradated until >30 BMI = no more Medicare pending graduation from a "weight loss bootcamp"). I can pretty much guarantee a huge amount of people would be totally not happy.

I could use similar harsh, if negatively over-generic, logic to justify the rule ("I'm not paying for the health costs of some fat ass who ate so many donuts in their life they're now having a heart attack on my dime!"). But such an approach will probably not solve very much anyways except make a whole lot of people angry. People would probably hold donut-eating parties in protest. :)

One other point. At present, the current state legislative trend on drug-testing "welfare" recipients is mostly limited to TANF, with occasional forays into Medicaid and SNAP. I have seen no proposals targeting SSDI / Social Security or Medicare. There's a reason I said "caste" -- right now, the context is not people in general that are accepting money from the government, but only the "wrong" people that are accepting money from the government.

How do you define "accepting money from the government"? I assume you're mostly referring to SNAP, TANF, and Section 8 but where do you draw your ideological line? Anyone accessing any Federal Assistance? Does SSI count? Tax credits? Pell Grants? Should drug users be denied access to tax funded infrastructure and emergency services?

Let's not forget tax-expenditures like the home mortgage interest tax deduction.