I think Kelo makes this possible.
I'd think that confiscating 401(k)'s goes against the Fourth Amendment protections of "The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated..."
Seizing one's 401(k) sounds a lot like seizing one's papers (stocks, funds, money funds, etc.).
-PJ