Barney Frank's view of what should be legal (via Glenn Thrush) could've come from John Stuart Mill. Frank says that marijuana and internet poker should be legal, as any harms they generate are harms to the user, and it's not the business of government to address self-regarding harms. However, banning various kinds of financial transactions make sense, because as we've seen, these transactions can pose major risks to parties who aren't involved.
I haven't found anything on this specific issue in Mill, probably because he wasn't thinking about a society where a few banks could blow up the global economy by pursuing profit in ways that generated massive systemic risk. And as far as I know, the economy of his day didn't allow for that sort of thing to happen. But consequentialist defenses of liberty need to be updated for dramatic changes in the way the world operates, and I'm guessing that careful study of the issues that arise in a huge and interconnected global financial system would get him behind a reasonably aggressive system of regulation.