I don't think I've seen a bug report about that. This is due to constructing the URL by string concatenations; using java.net.URL or java.net.URI, the brackets could be added automatically if needed (depending on the constructor used).
As a workaround, because you're working on a custom RunStyle, override RunStyle#getLocalHostName to add the brackets when the InetAddress is an Inet6Address.
Feel free to file a bug in the issue tracker. And it'd be awesome if you could provide a patch! (note that there's a similar issue in RunStyleSelenium I guess)