Create both a highlight annotation and a button over the text. The button should be completely clear. No text, border, or fill. Then use JavaScript from the console window to get the name of the highlight annot. Once you have the name, you can use JavaScript in the button action to turn the highlight on and off.
Thom Parker
The source for PDF Scripting Info
www.pdfscripting.com
Very Important - How to Debug Your Script