Add System lines
Add option to lines, to be system line. It means, it would appear in all parts.
Now, all lines are staff lines (except volta lines).
It could be same principle, as it is in text. You can choose between staff text and system text.
Ideal solution would be an option to all lines (in inspector).
Other possible solution is to add extra type of lines - system lines.
(At the moment, it is possible to "hack" it by using volta lines).