I've been noticing that the usual solution for adjusting the shape of a phrase mark:
\shape #'((0 . 0) (0.5 . 1.0) (0.5 . 1.0) (0 . 0)) PhrasingSlur
...doesn't seem to apply to instances in which phrase marks extend across multiple systems. At best I can find ways to modify the first half of the phrase mark at the end of one system, but not the second half at the beginning of the other.
I need the ability to modify both in order to fix collision issues. Is there a trick to modifying an entire multi-system phrase mark?