I agree with Bradley.
Patriot and Prowler already have secondary-express queues built. Timberwolf's

-exit could be used for FOL access. I think the exits of Spinning Dragons and Nile could be used with little issue.
The go kart ticket booth has a Gate Central sales terminal and doesn't see frequent traffic. They could sell the FOL wristbands from there. You could also include unlimited go kart rides as a part of the FOL fee.
With five rides offering the service for 7 hours it would probably cost the park $320 in additional labor per day meaning they would only need to sell at least seven wristbands at $50 a piece to make a profit.
I think it could be successful on just about every Saturday in July and October. By biggest concern is balancing the number of guests from each queue. I know in past seasons some haunt lines have been at a complete stand still for several minutes or more because those with Front of Line access were ALWAYS given priority. I think for every two groups taken from the express queue one group from the standby line should be allowed to proceed.