Disclaimer: I've never owned, used, or held one of these. I'm not likely qualified to give an answer.
But despite the lack of qualification I'll throw in my pair of pennies. I recall seeing a video of a commercial version recently that had pins used for setup. The pins, if I recall, were mounted on the fixed part of the tool at the centerline. This way you could probably use an indicator or reference surface on the part to get to a known position. Makes sense to me anyway, even if I'm just making it all up.
Well, maybe not a complete solution but I think if you get creative with an indicator you can probably figure out a workable solution. If I didn't have access to a really nice CNC lathe at work, I'd probably want a ball turner too.