I had a goal of figuring out a method of directly querying MP3 and Vorbis metadata from the Python scripts that power ICYG over this vacation. I think I've met it.
I finally found mmpython, a comprehensive Python library dedicated to querying media metadata. This was the first time I've installed a library into a Python instance, as opposed to dumping a single module into the lib directory, so it was a good learning experience for me. After installing it and looking at its invocation method in the supplied script, I quickly figured out how to use it to query a particular songfile for metadata info.
Currently, I ask ICYG members to sort their music directories by artist so that my song-picking script can parse those names to make sure it's spreading picks out between available artists. With this enhancement, I'll have the script query each eligible songfile for the artist information. I'll also be free to pick using other base metadata info if I want.
I say base metadata because apparently the free-form non-default tags we use for artist and purchase links aren't available through mmpython. That's OK though, since I extract that info out from the play log, not via a direct query of the associated songfile or its path.