note head shapes

[John Sankey]

Current plans are for two note head shapes to be implemented
(in about a month) by:
\property Voice.noteHeadStyle = "oval"  (default)
\property Voice.noteHeadStyle = "diamonds"

So, a natural way to add what Eric wants would be
\property Voice.noteHeadStyle = "empty"

BTW, for modern music (and for percussion), it would be useful to have
\property Voice.noteHeadStyle = "x"
where "x" could be any single character.