If this thread is still alive, my vote would be for access to the export functions:
audacity MyProject /exportWAV "Backing Vocals Track" /exportWAV "Lead Vocals Track"
audacity MyProject /exportWAV *
where the tracks were referenced by their track name and the files were written to the current folder using the track name. Refinements could be allowing * to denote all tracks, or maybe supporting wildcard matches or regular expressions.
Also,
audacity MyProject /exportWAV "Backing Vocals Track" /exportWAV "Lead Vocals Track" /exit
would allow Audacity to exit after all command-line operations had been carried out.