Cross-staff phrase marks

[Hilary Snaden]

Are Bezier curves practical for making inter-voice and cross-staff
phrase marks in LilyPond? It looks as though this could be quite
labour-intensive, so any pointers or examples before I start could be
useful.

It's for a piano piece where a long melody moves between soprano, alto
and tenor voices. The phrase marks aren't particularly long but they are
very curvy.

-- 
Hilary