I remember seeing this offered a few years back by a lab here in LA, can't remember which one. It was nice though, and I guess a lot of people would use it. I believe they were using an 11x14 horizontal enlarger with a glass carrier to sandwich the negs on, then printing them onto 16x20 paper. They called them "super proofs".
if you shoot 6x7, you can't proof all 10 shots onto an 8x10 piece of paper at once, so, therefore the 11x14 enlarger.