...answering myself in pubic for posterity

Seems I've been a bit of a fool and misread the bug report, and it is in fact the tracker-extrator-vorbis package that pulls out the metadata. Just a tad of confusion about which version to use.

Now I know what source to look at, you can clearly see that it doesn't do cover art. The patch that's supplied in the bug adds support for the deprecated method using COVERART comments. I'll get that working first, then look at pulling in the correct extraction code from Harmatten.