I don't know if this is close enough to what your looking for to help you or not. I use a java bookmarklet to Highlight specified search terms on web pages. Here is the java script. Just create a dummy bookmark in your bookmark bar, right click it, and replace the URL with the java in the address section. Name it as you wish.
Just wondering, why do you need to have that functionality? I'm trying to think of a practical use to have such a plug-in, but I fail to see one. Could you describe the usefulness the plug-in would serve?