Well it’s kind of obvious. But Spotlight can’t or won’t hand a file with a zero byte resource fork to a custom Spotlight importer.
I need to investigate if it is smart enough to consider a resource fork (or the 10.4 xattr metadata), I’ll post more on that later.
I found this because I was testing an importer I’ve written by “touching” a test a file and wondering why the heck my importer wasn’t being triggered.
However mdls will display the file system “virtual” metadata for a file.