Many thanks to those who responded.
It turns out that I had a case of brainfade - I had put the shell script
that came with Lilypond where it could be triggered by LilyTool in
jEdit, but not in a directory that was in $PATH. Hence a call to
lilypond from the command line did nothing.
FWIW, even though I'm now able to call Lilypond from the command line
and specify PNG output, I think that enabling the GUI version on Mac to
let the user select the output format could be a useful addition. I have
some other Mac users interested in Lilypond, partly because the output