I just downloaded Synergy on the recommendation of my fiancé as a replacement for Quicksilver, which I only ever used to control iTunes. I like the Synergy interface better than Quicksilver, and I really like the ability to customize the size, position and appearance of the floater, but there is one crucial (for me) element missing from Synergy: the ability to run AppleScripts.
I created several custom AppleScripts to allow me to skip back and forth in the current track by 5 seconds, 30 seconds, or 1 minute. They're nice for when I didn't quite catch the lyric that just played, or when I'm listening to a track with a "hidden" song at the end of a long silence. With Quicksilver, I can run these AppleScripts with global hotkeys, but with Synergy, I have to bring up iTunes and do it all manually.
I would really appreciate the ability to control these AppleScripts via Synergy, and possibly add more in the future. If you're interested in adding them in as default options, I can send you my AppleScripts, but for people who enjoy tweaking the controls, it might be preferable to build customization abilities into the interface.
As far as I know you already can do this without needing any third-party software. For example, check out this article, which describes how to add AppleScripts to the Script Menu, and then use the "Keyboard Shortcuts" tab of the "Keyboard" pane in the System Preferences to launch those scripts via a hot key.
Ah, one thing I forgot to add. Although you can't do exactly what you are describing within Synergy, at least in the case of the example you cite (missing the lyric) you might find the press-and-hold functionality to be useful: the normal "prev" and "next" actions apply when you hit (press-and-release) your assigned hot key combination, but if you instead press-and-hold you will cause iTunes to scrub backwards or forwards through the track until you release.
In any case, if you can't get the script menu thing to work let me know and we'll put a proper ticket for your request in the issue tracker.