No ledger for custodes beyond 5-line staff

[Matthew Fong]

I was engraving a piece of chant in modern notation recently, and
discovered that none of the custodes will draw a ledger for middle C (C4)
or A5, notes beyond the 5-line staff. These notes do occur in chant and
polyphony.

Is there a workaround for this?

----

\layout { ragged-right = ##t }

\new Staff \with { \consists "Custos_engraver" } \relative c' {
  \override Staff.Custos.neutral-position = #4

  \override Staff.Custos.style = #'vaticana
  c'1^"C4 custos missing ledger" \break
  c,1^"A5 custos missing ledger" \break
  a''1
}