Has this been tested in court? If your crypto is stored in a brainwallet, you can invoke fifth amendment rights to prevent that from being divulged.
Edit: Civil asset forfeiture already runs afoul of the fifth, and no one seems to care:
> nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.
"right to remain silent" is literally for that purpose. There are ways around it, if the government grants you immunity, but it's still far more onerous than freezing a bank account or invoking civil forefiture. Not to mention, disclosure of the key and/or wallet addresses may plausibly result in you incriminating yourself (eg. by proving that you bought drugs on silk road or whatever).