Well, Wikipedia probably wouldn't be vulnerable, but where individuals are involved he could get a default judgement in a plaintiff-friendly UK court for a no-show defendant in the US, with costs awarded (which is very common in UK cases), and then use that as leverage in the US. This technique has been tried against Usenet posters and worked well. Since Wikipedia content is produced by individuals who are often identifiable it would probably work as well on Wikipedia. He may have to demonstrate standing to sue in UK courts, but that could be as simple as selling books through Amazon.co.uk--which Wollmann does, as it happens.
Yes, yes, the situation is hopeless. We're all going to be sued into abject homeless poverty if we date to publish anything but the innocuous pablum known to Western Civilization.
We'll begin by renaming it the Nicepedia.