Boeing looked at doing the same sort of thing with the Large Cargo Freighter (aka Dreamlifter) back during the early part of the 787 program. On the surface it looked good - even at peak production rates the LCFs fly around empty half the time - but deemed it impractical and not worth the effort.
With the sharply reduced production rates for the 787, it might be worth Boeing/Atlas giving that a second look as well.