Sorry for not responding much sooner. I watched this space for a few weeks and then kind of wandered off. Thanks for your suggestions, Martin. I'll have a look at those projects. In the interim I just went back to using ffmpeg for logarithmic spectrograms. It took a bit of trial and error to figure out the gotchas in ffmpeg's implementation (there's no documentation about the effect of output image dimensions, but it's extremely sensitive to that). On the plus side, it seems like someone may have...
Spectrogram - Log/MEL/Bark frequency scaling options