This is what happened with Ross Ulbricht, right? The FBI seized more than 200K coins from his wallet and later they auctioned all those coins. So my guess is that it is possible for the courts to confiscate the coins.
They seized his live unencrypted computer. That's how they accessed the coins. If he'd managed to shut down and purge his computer before being arrested and refused to let them know how to get into it they could not have gained ownership of those coins.
No court can confiscate a properly set up wallet. It's not technically possible. However since they have jurisdiction over you they can force some extremely unpleasant things on you yourself until or unless you agree to let them in.
If they never know about it then they can't force anything about it on you. You can bet your bippy they'll be watching any financial movements if they suspect you of something very closely though. In that case you need to stay away from any fiat through an exchange or traceable expenditure such as via Bitpay.