I now have a working script which populates WGT's forum navigation pages with last poster usernames.
It doesn't insert the usernames as fast as it would if WGT did it, but then I don't have access to their database (thankfully), but the speed is I feel, acceptable.
To use the script you will need the browser extension Tampermonkey (linked to below), and once you have that installed, you can click on the script link (also linked to below), to get that installed into Tampermonkey.
Clicking the script link will open Tampermonkey's script install page where you can view the script and choose to install, by clicking the install button, or cancel the install, by clicking the cancel button.
The Tampermonkey extension puts an icon on your browsers toolbar from where you can switch the script on and off. I would advise switching the script off occasionally just to see if WGT have fixed this issue.
Last Poster Username Script.
EDIT : One thing I have overlooked on this is it takes away the bold text on the threads you haven't viewed, so you won't know what you have and have not looked at. Looking into a solution.