jamietre - 3:12 pm on Jul 14, 2011 (gmt 0)
Not sure if you are still looking for a solution but I wrote a jquery plugin to do exactly this called Image Mapster: [outsharked.com...]
You can provide it with an alternate image of the same dimensions as your image map, and it will use the alt image as the source to render the highlights.
If you don't want the weight of the plugin (it's about 28k compressed) you could take a look at the source to see how the alt image part works, it uses canvases to extract the contents of one image inside the boundaries of the image map "area", and renders it with a mask.
Since canvases aren't available in older Internet Explorer browsers the plugin falls back on an opacity highlight for those browsers.