I'm not sure how well this fits in with what you're looking for, but the Scripting Bridge framework is always and option. The example below just changes the comment on which ever track is currently playing, but you can use iTunes.h
and Scripting Bridge to do basically what ever you want with iTunes and its tracks.
#import <ScriptingBridge/ScriptingBridge.h>
#import "iTunes.h"
iTunesApplication *iTunes = [SBApplication applicationWithBundleIdentifier:@"com.apple.iTunes"];
[[iTunes currentTrack] setComment:@"I just added this cool comment to the current track!"];
There are recently generated copies of some of the Scripting Bridge headers on github, but I recommend that you compile your own to make sure everything is up to date. Instructions can be found here https://developer.apple.com/library/mac/DOCUMENTATION/Cocoa/Conceptual/ScriptingBridgeConcepts/Introduction/Introduction.html
Basically, to generate iTunes.h
you'll need to run the following command.
sdef /Applications/iTunes.app | sdp -fh --basename iTunes