It's probably worth noting that I'm using LoadUI 2.7.0. Selenium appears to be available by default, so have removed the jar from the ext folder. Still the same result. The Web Runner appears to be working with a particular URL, although it's not logging any output, so can't verify. I've an inkling that this is a proxy issue because nothing at all is registering in Fiddler. There doesn't appear to be any way to configure a proxy from within LoadUI.
I can't see anything out of the ordinary in Process Monitor, I can see the script being loaded and the Geb and Selenium drivers loading, so it IS doing something, but what that something is I don't know.