Horizontally-Centered Stems for re, mi, and ti

[Jason R . Fruit]

I was looking at Jesse Aiken's _Christian Minstrel_, the original source of 
Aiken-style shape note heads, and I saw that he centers the noteheads on the 
stems for the symmetrical shapes, i.e. re, mi, and ti.  I think it improves 
the recognizability of the shapes, and I'd like to do it in my shape-note 
music.  I can't figure out how to make it happen only on those scale 
degrees, though, other than to do like this:

\version "2.14.2"

\relative c' {
  \aikenHeads
  c
  \once \override NoteHead #'stem-attachment = #'(0 . 0) d
  \once \override NoteHead #'stem-attachment = #'(0 . 0) e
  f g a
  \once \override NoteHead #'stem-attachment = #'(0 . 0) b
  c \bar "|."
}

What I'd like to be able to do is:

\relative c' {
  \aikenHeads
  \centerReMiTi
  c d e f g a b c
}

Is there an example of scale-degree-sensitive code I can look at?  I can't 
seem to find where aikenHeads is defined.

Jason