Well I know from my own personal experiences that a letter addressed for the attention of the Captain is usually received positively. Just make sure you outline your reasons and all being well you should find yourself in the J/S. However, be aware, it's always the Captain's call and if your flight happens be a training flight then expect to be disappointed. However, a letter has always worked for me. BA 777 LGW-DFW/ DFW-LGW twice and various B737 Ryan Air/ B.Midland flights in and out of Dublin.
GOOD LUCK
C.N.