replace a notehead with a centered text

[Paolo Prete]

Hello,
how can I replace a notehead with "some text" centered on the stem?
If I use:
customNotehead  = {\once \override NoteHead.stencil = #ly:text-interface::print 
 \once \override NoteHead.text = \markup {  \fontsize #6 "some text" }}
the notehead is always at the left or at the right of its stem...
thank youP