Seizing it would also have the secondary effect of a mass exodus of investment capital from the country, like capital flight in China.
A more reasonable way is to seize it is through taxation, either by a wealth tax or tax on unrealized gains, but it would be very difficult to implement.
Capital flight to where? The US economy is still the largest in the world, there aren't enough assets in the rest of the world for that capital to buy.
Plus, the US government could just implement capital controls overnight (or better yet, in the night before), then the money would have nowhere to go.
A more reasonable way is to seize it is through taxation, either by a wealth tax or tax on unrealized gains, but it would be very difficult to implement.