Currently Gereqi uses tagpy for tagging duties. It's a small and simple tool but sadly is lacking in file format support. This seems to be due to it being a wrapper for taglib so allows a little room for expansion.
There are plenty alternatives out there. Gstreamer has support for tags but documentation for it is elusive. The package I shall be moving to is Mutagen. Quod-libet and Ex-Falso use it. The latter being well-known for it's tagging features.
As I am trying to keep dependencies to a minimum Mutagen is a good choice as it is a needed dependency of libgpod, which somewhere down the line Gereqi will need for iPod support.
So, if I can get around this refactor issue, Mutagen should be coming up soon.