Thanks so much David and off list repliers.
I'm deeply grateful for the generous help on this amazing list.
Here's what works nicely:
\version "2.19.82"
harmonicNote =
\tweak NoteHead.stencil #ly:text-interface::print
\tweak NoteHead.font-size #1
\tweak NoteHead.text #(markup #:musicglyph "noteheads.s0mi")
\etc
{
<g' \harmonicNote c''>4
}
Andrew