Ben:
I'm wondering if there is an easy way I can use an effect or similar
function to display the waveform of an audio track in the compositor
output (and final render).

The ffmpeg plugins that I "think" might get you what you want are all commented out in {your cinelerra path}/bin/ffmpeg/
plugin.opts file.  When I uncomment these to test, they all come up with "Input/output error" and am assuming that this is because the required data is not available at the time needed.  I will have to ask GG to be sure though or to see if there is a workaround using a pre-Opts file, and he is unavailable right now.

If you want to test this yourself you can do so by modifying the plugin.opts file in the cinelerra bin ffmpeg directory and uincommenting and removing the #-s at the end of the line for ahistogram...showcqt... or whatever.  Then delete Cinelerra_plugins in your $HOME/.bcast5 directory so that the plugins are reloaded.  Start Cin from a window to see the messages instead of an application icon.    gg/Phyllis

------------------  Error messages I get in Cin-GG ----------------------------
PluginFFilter::new_ffilter(ahistogram)
  err: Input/output error
PluginFFilter::new_ffilter(aphasemeter)
  err: Operation not permitted
PluginFFilter::new_ffilter(showcqt)
  err: Input/output error
PluginFFilter::new_ffilter(showfreqs)
  err: Input/output error
PluginFFilter::new_ffilter(showspectrum)
  err: Input/output error
PluginFFilter::new_ffilter(showspectrumpic)
  err: Input/output error
PluginFFilter::new_ffilter(showvolume)
  err: Input/output error
PluginFFilter::new_ffilter(showwaves)
  err: Input/output error
PluginFFilter::new_ffilter(showwavespic)
  err: Input/output error