One option is to use a servo reverser. Not unusual with airplanes. Here are a couple examples:

http://www.futaba-rc.com/radioaccys/futm4150.html

http://www.rcdude.com/servlet/the-19...rser-V2/Detail

Or if you want to just rewire the servo, something like this:

http://www.youtube.com/watch?v=EK_FdRTenl0