We recently had customer using 5D MK 3 with 24-70mm (shooting at 24mm) on M1L w/RD16. While we are unable to confirm settings customer claims he was unable to achieve full rotation upwards in shooting full spherical panoramas.
If you are customer with same camera/lens there are options:
1) M series Nadir Adapter will add sufficient hight to vertical rail but at the cost of having to purchase the item.
2) Tweak your workflow to get around this constraint. You could for example slide your camera rail slightly to allow for full rotation up and let it settle back to shoot zenith. I did this with NN3 for years before the NN5 came out. The zenith hole to cover would be small enough that parallax shouldn't be an issue.
3) Tweak your workflow whereby shooting with enough tilt up will capture the zenith while shooting upper row.
Nonetheless we have had a couple customers share similar concerns.
It might be worth considering making an Arca-swiss style "block" so users can add height to vertical rail by mounting block onto the lower rail. You could even make blocks with various heights.