After I sent the letter, I got a nice response from the director thanking me for taking the time to write out my concerns and letting me know that it had been forwarded to the director of risk management.
When I mentioned it to my obstetrician, she thought I was being silly and should take it less personally. "Sure, it would be fine for you to make that decision; you're informed about your health. But lots of people don't follow their doctor's instructions." (That made me grumpy.)
Today I got an e-mail from the director of risk management, informing me that the policy has been changed and now they'll just use the regular waiver that they use for everything else.
So I guess I'm going to prenatal yoga next Sunday!