where to buy a friction roller

Your rack mounted installation sounded pretty good to me Toby. Especially if you can get help creating the sprocket plate which will bolt to the disc brake hub. I have never riden a rack mount bike and always wondered about the high center of gravity. A lot of people do it, so it must not be an issue. I would definitely make my own rack out of heavier material and brace it securely because this assembly is going to weigh more than a China Doll with a roller. I think you've had enough experience to make this work. Best wishes to ya.