Thanks, Jack! I wasn't expecting this level of help, and I really appreciate it.
I've gotten your plugin working on a test page, but I'm running into one issue:
I can apply tweens to all pseudoelements on the page using the getRule(":before") syntax. But for some reason, getRule is failing when I try to find a specific instance of one, such as getRule(".myClass:before").
I've put together a quick test page to show what I mean.
https://dl.dropbox.com/u/56154/gstest/pseudoelement_tweens.html
There are three tweens being called on the page. The red box tweens to yellow, all :before elements have a yoyo-ing tween of their border width, and theoretically the text in the :after element should be tweening to white. But the last one isn't working, and the javascript console reports that getRule is returning undefined.
Am I making an obvious error?