Hi Brian,
And do:
\version "2.20.0"
whiteoutHalfNoteAlter =
\tweak stencil #(lambda (grob)
(grob-interpret-markup grob #{
\markup
\override #'(filled . #t) {
\combine
\path #0
#'((moveto 0.000 -0.250)
(curveto 0.000 -0.200 0.060 0.210 0.350 0.365)
(curveto 0.510 0.460 0.700 0.545 1.050 0.545)
(curveto 1.200 0.545 1.380 0.500 1.380 0.270)
(curveto 1.380 0.120 1.290 -0.140 1.120 -0.300)
(curveto 0.870 -0.500 0.520 -0.542 0.320 -0.542)
(curveto 0.200 -0.542 0.000 -0.530 0.000 -0.250)
(closepath))
\with-color #white
\path #0
#'((moveto 0.000 -0.250)
(moveto 0.110 -0.270)
(curveto 0.110 -0.380 0.210 -0.435 0.280 -0.435)
(curveto 0.370 -0.435 0.600 -0.300 0.850 -0.135)
(curveto 1.100 0.030 1.273 0.140 1.273 0.270)
(curveto 1.273 0.350 1.200 0.440 1.090 0.440)
(curveto 1.040 0.440 0.850 0.350 0.600 0.185)
(curveto 0.200 -0.080 0.110 -0.150 0.110 -0.270)
(closepath))
} #})) \etc
{ \whiteoutHalfNoteAlter c'2 2 }
Cheers,
Pierre