Yeah, it doesn't support that.

I think the next step would be that when you add a product to the registry there's a checkbox like "let people buy this from other stores" which is default checked on amazon/babys-r-us and default unchecked on etsy.com. Then in the registry display you would see multiple offers for one product. Then there would still be the challenge you describe but it wouldn't be a UI problem, it would be a data challenge.