It's possible that the coins are held up in a court order/gag order but it is in no way probable.  Remember that Ukyo's

• traveled outside the country
• distributed some of the money back to users
• been pretty active on the forums asking people not to act

Ukyo would be forbidden to do all of these (or at least told to sit tight by a lawyer) if he was involved with or under some kind of investigation.  There's no way in hell Ukyo could have done any of those three if he was under the national security letter gag order.

Instead, we've got a guy who has an incredible thirst for capital:

• owns a profitable securities exchange
• owns a profitable payment processor
• still needs to finance himself by issuing personal debt

Why does Ukyo need so much capital?  The probable answer is that he's pissing it away on gambling or bad investments.