I have seen such a thing once. Didn't buy it, because things weren't looking good for Polaroid at the time, but maybe I should have. It must have been replacement for the full back, because anything that can hold a medium-format Polaroid film pack isn't going to fit under the 6.5x9cm groundglass. You might check with Bob Salomon to be sure.
If you're really desperate, one solution might be to get one of those Chinese Linhof 23-->Hasselblad V adapters that you can find on eBay and use a Hasselblad V Polaroid back.