For those of us caught out by that ellipsis in the URL, I've modified the post so that the full URL is displayed in the post and can be copied directly from it instead of having to hit the Quote button and copy it from the editing text box
I wanted something simple for Chrome, so just made a quick bookmarklet.
I named my bookmark OwlChecker, but feel free to name it whatever.
Next go to step 4 within OLWLG and click on your new bookmark. The script will give an alert once it has finished loading.
Once loaded, the first checkbox clicked inverts normally and then is disabled (to help track which was the first one clicked). The second checkbox clicked triggers the script to invert all checkboxes within range of the two. This way you can quickly check a whole row, column or any other rectangular range.
That's it. Should work fine on all browsers that are supported by jQuery. Do note that the script is currently hosted on my Dropbox account, so should you not have access to Dropbox where you are then it will fail to load. You can download the script and host it yourself where ever you like: download OwlChecker
Edit: My web host went MIA so I've moved the script back to dropbox.