A letter?
Wings folded, perhaps if you were to obtain a letter from your Dr. stating that you are perfectly capable of undertaking the tasks associated with an ABP, you could, on encountering the first FA you meet, proffer the said letter with the brief explanation that you are perfcetly willing and able to act as an ABP, should the cabin crew require one.
Bear in mind that some people actually pay to be sat at emergenct exit rows so your suggestion will not alwys be acted upon. Where it is a simple overwing exit type of row then it may well be that they have already been taken by other, apparently suitable ABPs.
If I were you, I would find a seat that is quiet, next to the window and hopefully within a row or two of the exit. Then snooze away the flight with your i-player.