Not familiar with Zcash specifically, but it should work the same as Bitcoin. You cannot initiate a transaction without the (password protected) private key of the wallet containing BTC. If the victim of civil forfeiture refuses to provide the password and private key there is nothing that can be done to acquire the money. This is good because it changes the game from the "government takes your money and you have to sue to get it back" to "government has to prosecute to get your money in the first place".

It doesn't prevent some vague yet menacing government agent from breaking your kneecaps in order to get the password, but no security system has been able to plug that hole yet.