Not overlays, but solid lenses.
For instance, SeaVision (http://www.seavisionusa.com/), make their corrective lenses out of plastic. Their web site says the lenses are "made from the highest quality materials (CR39) that are stronger, and more efficient than glass."
I just looked up CR39; Wikipedia says it's not polycarbonate, but "a plastic polymer commonly used in the manufacture of eyeglass lenses."
Plus the SeaVision web site says, "Please do not clean your SeaVision Mask with any abrasive substances (toothpaste, scrub cleansers) this will damage the anti-fog quality." I think what it'll actually do is scratch the lenses, and/or damage any coating they put on.
As an aside, weight does get to be an issue when the (glass) lenses are as thick as mine are, with a -9.5 correction. They make my mask quite negatively bouyant. I use a Marsoops neoprene strap that compensates for the negative bouyancy, making the whole mask pretty much neutral -- I figure if I lose it, it'll just hang there in front of me, and I'll at least see the lime-green strap to find it.Plus the strap turns inside-out to double as a soft padded case to protect the mask when I'm not using it -- not a minor point for me.
I went ahead and added a note regarding non glass lenses. Thanks.
Pete