Marking cross-staff unisons

[Michael J. O'Donnell]

I have a 4-staff SATB choir score, and I would like to mark unison
sections with some nice vertical bracketing. The bracket style for
arpeggios looks good, but the cross-staff arpeggios appear not to
support it (the documentation says that cross-staff parentheses are not
supported, and it appears that this applies to brackets also). Grid
lines are a bit ugly for this purpose. I couldn't find anything in the
archive nor snippets (but I don't always think of the right search
terms). Ideas?

Thanks,

Mike O'Donnell