Copying data to the comment field
Unfortunately, iTunes does not support a number of fairly standard fields—most prominently probably musical key. To help solve this problem, there have always been beaTunes plugins that copy certain information to the comments or grouping field. Those plugins can be installed via the beaTunes preferences. However, it seems that many people want to have more control over what exactly is written to the comments field. Some prefer OK-Notation, others want musical keys, yet others want to come up with a completely new notation.
While it is possible to code up a new Java plugin for beaTunes that suits your personal needs, many people find this too much trouble. This is why I introduced beaTlets a while back. beaTlets allow you to script beaTunes plugins using Groovy, JRuby, or Jython. In other words, instead of installing, Java, Maven, an IDE etc. you just have to code a single class in whatever text editor you like (TextMate, JEdit or Notepadd++ come to mind), drop it into the beaTunes plugin directory and restart beaTunes.
But before beaTunes 3.5.17 it hasn't been possible to script a SongAnalysisTask. That's the kind of plugin that works like the KeyToComment plugin you might have installed via the beaTunes preferences. It shows up in the Analysis Options dialog, when you click on Analyze. So... if you're interested in controlling how the key field is copied to some other field, please check out the new sample code for Song Analysis Task beaTlets. When rolling your own, please keep in mind, that any error messages are dumped to the beaTunes log, the file name of your beaTles must reflect its class name, and that no two entities can have the same name (you might have to uninstall plugins with the same name).