The direct tax clause of the original constitution, even as modified by the sixteenth amendment, probably forbids such a thing at the federal level. So it'd likely take a constitutional amendment.

That said, I'm not quite as pessimistic as you. Between consumption taxes and estate taxes (which are about the only wealth tax we have) we could get at most money if we had the will to do so. The estate tax is filled with holes because Congress wants it to be, not because of any inherent complexity or definitional difficulties.

There is an ultimate tax avoidance strategy of renouncing citizenship and living and consuming abroad. I would impose a lifetime ban on entry on renouncers, but beyond that I think it is fair game. If you aren't an American we shouldn't be taxing you.