About 40% of patents are invalidated in the course of infringement suits. When a patent is upheld, about half the time the court finds that the alleged infringement did not occur.  So there would be a valid patent to seize in about 30% of cases in which a patent troll loses its suit.
It's a start. I don't think losers' penalties would fundamentally change the financial calculus for patent trolls, however, because 97% of patent suits are settled before they go to trial.  Doubtless a similarly large share of threatened suits are resolved before a suit is even filed. What needs to change is the calculus for the troll's target of paying up versus challenging the patent in court.
Here's a crazy idea. Perhaps patent-holders should be required to register licensing income, as well as infringement complaints, with the patent office. A patent's validity would automatically be assessed by a court if the licensed value or the number of complaints passed some threshold. Since so many patents are invalidated, this would encourage patent-holders to seek royalties or threaten litigation in fewer circumstances.
Alternatively, a similar system could be used as the basis for enabling class actions to challenge a patent's validity. Merely publicizing a patent troll's actions would help its targets organize to share litigation expenses. Wouldn't it be a wonderful thing if we began to see television ads asks, "Have you been threatened with a lawsuit and strong-armed into paying licensing fees on a patent covering ..."
1. Michael Heller, Gridlock Economy, 2010.