Plausible in the sense that you would accept having a "joint" account with the govt. People don't do that now so it seems unlikely they'd be very willing later.
Plenty of people, in a sense, have this, at least in the U.S. It's called a bank.
Government thinks you're a criminal? Boom, your account is frozen. They think you owe them something? Boom, they take the money out directly. The fact that they don't do it that often doesn't mean it never happens. And most people still wouldn't DREAM of not having a bank account, or worse, banking with some *unregulated* bank that doesn't do what the government tells it to.
I've even heard the sentiment expressed in this forum that letting government take a little bit of control of bitcoin in some form or fashion is a good thing because it lends bitcoin legitimacy! To my mind, the idea of there being a push toward government-controlled (read: "regulated") multisig accounts isn't just plausible, it's a question of "when", not "if."