When I got my RS7, one of my concerns was it only had 2 rear seats. With 3 kids, it was a problem but I bought it anyway with the hopes of being able to convert it to 3 seats. A full A7 black leather rear seat came up for sale on ebay and I jumped on it.
After having done the work, i will say it is a direct replacement for the RS7 seats. The A7 seats came with the built in third seatbelt (i just need to order the bottom connector to make the third belt complete). It doesn't have the diamond stitching anymore but that doesn't really bother me as much as not having room for my three kids.
Below is a pic of the finished backseat, the "dents" in leather will come out after it sits a while.:

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