wayback,
have you told your sensei why you do not breakfall? if not, do it now! i have trouble picturing an instructor (or any Aikidoka) purposely throwing someone with a medical reason not to fall. if he's unaware, he may just be trying to 'help' you over your fears. I myself much prefer breakfalls to rolls; my last dojo's instructors, knowing this full well, would use me for uke in zenpo techniques to encourage rolling. My first dojo put red tape on an injured body part/gi...my current one uses duct tape (not quite as good since the same tape is used to repair gi's at times, causing some confusion)