Changing ottava text

Internally, \ottava sets the properties ottavation (for example, to 8va or 8vb) and middleCPosition. To override the text of the bracket, set ottavation after invoking \ottava.

Short text is especially useful when a brief ottava is used.

\version "2.24"

{
  c'2
  \ottava 1
  \set Staff.ottavation = "8"
  c''2
  \ottava 0
  c'1
  \ottava 1
  \set Staff.ottavation = "Text"
  c''1
}