A court could simply order them frozen, and expect the holder to comply. If the court deems that too dangerous, they would order them transferred to a third par

A court could simply order them frozen, and expect the holder to comply. If the court deems that too dangerous, they would order them transferred to a third party guardian (usually the local sheriff or similar, but they might make a different arrangement for something unusual like bitcoin). Possibly "without notice", if "dissipation" seems likely if the holder knows it's coming. Pretty much the same as you'd do with cash or other physical assets. (Transfer is necessary; seizing the private key isn't secure against copies. The FBI is known to have moved the Silk Road money to their own wallet: http://blockchain.info/address/1F1tAaz5x1HUXrCNLbtMDqcw6o5GNn4xqX http://blockchain.info/address/1F1tAaz5x1HUXrCNLbtMDqcw6o5GN... )