Perhaps just use another browser without pop-ups blocked for that access. Then you won't have to fiddle with the preferences - you just need to remember to use that browser for "pop-up block" problem websites.
You could even make the Fatcow login page the homepage for that browser.
You could use an image of Fatcow to change that browser's icon to remind you.
...If you use your right hand for selecting Command>Shift>K then it's not much of a problem if you use your left hand for click. I'm assuming you're using a trackpad but if it's a mouse then maybe not!