Whenever I've seen it brought up it's always that it would require a constitutional amendment (specifically in the US). The federal government isn't allowed to do taxes that way under the current system (Article 1 Clause 2 Section 3).
Hell, the feds weren't even allowed to do a straight income tax until the 16th amendment.
Yes, I mostly see people saying that laws don't matter because billionaires are beyond law at this point. Who knows, perhaps that's astroturfing by troll teams employed by billionaires to try and discourage the masses. As you said, they've already got plenty of lawyers to work on it from the constitutional end - including 6 of the lawyers on the Supreme Court.
Huh. Weird. Well, I don't see it brought up on HN all that often (once a month tops?), so we must hang out in different circles outside of here.
Anyways, you have a good one.