When we went last time, our family physician wrote the letter. The thing is, he's a great doctor, but tends to babble a bit, and his letter got, well ... long. This time my wife asked if she could write the letter - on the idea of saving him time, but also being more concise - and he agreed to sign it (after reading it over, of course.) This way we can explain exactly why we would need the pass. (In our case, waiting causes DS to meltdown.)

I don't know if that would work for you, but it might be something to consider.