Papervision - how to make irregular planes into buttons

Hi all, I’m not a regular here, but I hope you don’t mind me dropping in with this question …

Here’s a demo showing how far I’ve got:

danwashere.com/dev/papervision/interactive_cows.html

What I want is to be able to click on any of the three planes and make the corresponding red dot show or hide. It works on the top most cow - try clicking on it. But you can’t click on the other two.

I’ve also tried this with transparent PNGs with the same result.

I think that because it’s really a rectangular plane with some transparent bits, that it blocks the mouse click from getting to the planes below.

What would be a good approach here? Do I have to create these via collada or somesuch to make them into lots of smaller planes? Or is there something I can do with what I’ve got?