Possibility to lock feature line's grade would help us a lot.
By changing the grade of interim (incoming) feature line grade you determin the start of the locked grade feature line, but change also the endpoint elevation of it by the fixed grade.
This would function as a zero length grading (which we don't have as well).
In the light of instability of gradings - gradings except infills get corrupt when you edit the feature line it's based on - this option would help us a great deal.
Cheers
Robert