IANAL but wouldn't it be easier to convict on mail fraud first. Corporate "accounts receivable" type scams have been convicted before of mail fraud. The scammers send an invoice to a corporation, and if it looks routine, it gets paid without a second thought.

Sooner or later, some patent troll is going to be dumb enough to send out "Unpaid License Fee" letters and set a precedence on the extortion charge.