Not sure if you're North or South Island, but a Tom Thumb Screwdriver (Little Screwdriver) in the boardies pocket would work.... Or, dare I mention it, Boardies over Wetsuit !!

Or on a necklace in the wetsuit, with little screwdriver.... But yeah, the FCS Key is easier...
Just thought of 2 things.... somewhere that sells FCS Single Fins should have the screw.... Or hunt around for a nice little screw driver....
Email for McTavish;
[email protected]+61 2 66 856 736
http://www.mctavish.com.au/contact.phpAnd check out this lovely red rocket !!
boards.mctavish.com.au/boards.asp?modelid=9&storeid=1