Hi Dave,
if you use ABC and the parser, the short answer until the next version
is: you can't.
if you use the Music model, add objects such as Note, KeySignature,
you can set the clef to KeySignature.setClef(Clef.BASS) (or something
like that).
The rendering knows how to print a bass clef.
I'm working hard on a new ABC parser which understand clef, voices and
a lot of things.
The Music object structure is quite ready for multi voice (e.g. piano
2 staves treble/bass clef), the rendering no.