How to adjust the Y-offset of volta spanners?

[lilypond]

I have a first ending at the end of a line, which is nicely
spaced (vertically).

The second ending is at the beginning of the next line, and the
right end of the spanner is above a rehearsal letter (/mark
/default).  There is more space than necessary between the
rehearsal mark and the second ending volta spanner, and indeed,
more than necessary under the entire second ending spanner.

How might I adjust the Y-offset of the second ending volta
spanner while leaving the first ending volta Y-offset unchanged?

Early attempts have included:

\once \override Score.VoltaBracketSpanner #'Y-extent = #'(0 . 0)
\once \override Score.VoltaBracketSpanner #'no-alignment = ##t
\once \override Score.VoltaBracketSpanner #'Y-offset = #10
\once \override Score.VoltaBracketSpanner #'extra-offset = #'( 0 . 50 )

placed either at the beginning of the second ending, or at the
end of the first alternative, but nothing seems to budge the
volta spanner.  Is it possible to control the Y-offset of two
volta spanners independently, and if so, how?

Thank you!

Jim