Kensington Orbit has a wheel that surrounds the ball, and this wheel scrolls the page.
Doesn't help with the symmetry thing, but functionality-wise it's great.
Thanks, Kensington Orbit is my daily driver, absolutely love it!
I'm actually planning to use two of them for the build. I'd like to have 6 axis of control: right trackball move mouse, right scroll wheel verticle scroll, left trackball horizontal and verticle zoom, left scroll wheel horizontal scroll. might seem a little excessive but these are all functions I use a ton in my DAW so having them as dedicated controls instead of with modifier keys would be pretty nice I think.
Another thing a little unusual I'm thinking of trying is having the left/middle/right mouse buttons mapped to keys on the left half of the ergodox. Might make click-dragging actions nicer instead of having to hold the mouse button and move the trackball at the same time with right hand.