That is a hard one. I suffer from a rare autoimmune disease that has given me lots of problems that are not necessarily visible to the casual viewer. Perhaps when the pain is at it's worst you could wear gloves. Make them a bright color like red so they can't help but notice. It might make people reluctant to grab as quickly.
I wear a hat almost everywhere but my own home as sunlight and florescent light may could increase my symptoms. I volunteer with the quilters at our local Senior Center. One of the ladies after months finally got bold enough to ask me if I was bald under my hat. You might try wearing a small sign with a hand that says "please don't touch" or just "ouch". If the person is worth knowing they should be more than willing to greet you in another way. Over the last ten years my hats have been a subject of many a lively discussion. I get lots of compliments and lots of questions. It gives me an opportunity to educate others about my disease or them an opportunity to tap that compassion that makes us human.
Unfortunately there are a lot of "dumb clods" in this world. One of our quilters is 90 and has a frozen shoulder. This one guy who thinks he is God's gift to the females of our world always gives her a big hug every time he sees her. Having had a frozen shoulder several years ago I can imagine how she feels. She is also thirty years my senior and very frail. I may have to slap some sense into him the next time he does it in front of me.