I agree that no case to date is on point. Although the proposition of regulating so-called "inactivity" may not be so far fetched as it seems on the surface.
As the district court in Michigan noted, in Heart of Atlanta Motel v. United States
, 379 U.S. 241 (1964), the SCOTUS held that the Commerce Clause allowed Congress to compel those choosing not to engage in commerce to, in fact, engage in commerce. Of course you could attempt to distinguish that case on the grounds that the hotel owners were already engaged in the commercial activity of running a hotel. But I suppose if we accept the Michigan court's premise that, as humans who are always in need of medical care, we are already engaged in the commerce of the health care market, then it follows that our decision to buy or not to buy health insurance does have an effect on interstate commerce and is thus subject to regulation under the commerce clause.
What complicates this health care issue is the fact that the SCOTUS has never ruled that the test for whether something can be regulated by Congress under the commerce clause is whether that something is an activity or not. (It's always been assumed that it must be an activity). Rather, the test of whether something can be regulated by the commerce clause is simply whether it effects interstate commerce. (going back to Heart of Atlanta Motel
Again, as far as I'm aware, there is no Supreme Court case squarely on point that could settle the issue of whether the "thing" that effects interstate commerce MUST BE an activity as opposed to an omission.
One thing's for sure though, the SCOTUS has to grant cert here. There's no way this can be left to the circuits.
Boils down to Lopez in my mind. The test is whether or not this is regulating an activity that substantially affects interstate commerce.
Both Liberty U. and Florida (first one said constitutional, second said not) framed the issue that way. Is not buying health insurance an "activity"? If it is, it probably can be mandated. If it isn't, then it probably can't (under Lopez, anyway).
Liberty U. said basically said that the decision not to purchase health insurance is an activity and therefore it falls under the Commerce Clause.
Florida said that it is, almost by definition, inactivity (not buying insurance) and that Congress's power didn't reach that far.
It's not really Wickard/Gonzales because those cases, while regulating purely intrastate activity, were at least regulating activity (growing and consumption of wheat and marijuana, respectively). This isn't doing that. To play with Wickard a little, it would be more like the government mandating that you grow corn on your land (ignoring any seizure arguments or w/e; let's pretend we're solely in Commerce Clause land). I'm betting on 5-4 against.
By the way, the beautiful thing about the South Dakota law is that the legislature is trying to prove that they can't mandate the purchase of a firearm when, as a state government, they probably can.