I think it’s a bit of a fiddle.
The player needs to be able to turn the ball from left to right, because pressing D and rolling right wouldn’t make sense, when you’re in the ball. Maybe.
Anyway, if you make splines with components at the ends which can be scanned for with a sphere trace. That might work.
When you get to the end of a spline, sphere trace for these markers and change splines accordingly.