\version "2.19.0"

% changing the stem using Bravura font

\include "../smufl/definitions.ily"

Zstem = { \override Staff.Stem.stencil = #ly:text-interface::print
 \override Staff.Stem.text = \markup { \smuflchar ##xE1F3 }
}
  
\score{
 \relative c'{
  %\stemUp
  \Zstem
  f4 b c e
}}

% more generic example

newStem = { \override Staff.Stem.stencil = #ly:text-interface::print
 \override Staff.Stem.text = \markup { | }
}
  
\score{
 \relative c'{
  %\stemUp
  \newStem
  f4 b c e
}}