Modifying Stencil for Clef

[Richard Shann]

I've been following up the idea of using markup as stencils:

http://lilypond.org/doc/v2.18/Documentation/notation/modifying-stencils

and have got it working for NoteHead and for Dots (which was my use-
case). But trying to generalize to more layout objects I ran into
trouble. This I expected would replace the change of clef with a "T"
but it does not:

\version "2.18.2"
{ 
  c'
  \once \override ClefModifier.stencil = #ly:text-interface::print 
  \once \override ClefModifier.text = \markup{T}
  \clef bass 
  c'
}

Any ideas?

Richard