I strongly suspect that he wants people suing for USD entirely
USD?! This advice doesn't make sense unless you have USD on MtGox yourself and wish to buy cheap coins. The price in e.g. JPY is constantly 2.5% above the price in USD, and a currency which is much more liquid inside Japan, so why not JPY? Or EUR? The price in EUR is constantly 2.5% above the price in USD as well. And since all movement of USD has to go through US banks, you may bump into the USD liquidity problem MtGox has had for a very long time.
The answer from MtGox when people have threatened with lawsuits to get their USD out has been:
03:03 <@MagicalTux> x, a class action in the US to force us to do something the US govt prevents us from doing might not lead you very far
And AFAIK none of the lawsuits have been successful so far. So why lose 2.5% extra when selling to get a currency which they have problems delivering? This just doesn't make sense to me. EUR should be just as good as USD, and more liquid. JPY even better. So why USD?
To be clear, the attorney said he wanted to sue for USD over BTC. I think what he meant was that he was willing to sue for any fiat currency, not specifically USD. ...just not BTC.
As for MtGox claiming that a US class action wouldn't work - fine - this isn't a US class action suit. This is a Japanese suit against a Japanese company, and their funds in a Japanese bank. This has nothing to do with the US gov't.