I've had the rear seat belt dilemma too as I have two small children who were always at me for a ride in the Alpine. I couldn't figure out a way to fit lap sash belts, so I decided to just fit lap belts figuring that was better than nothing.
Daughter promptly went face first into back of my seat while I was backing out of the driveway, so I decided the lap belt idea was no good.
So then I tried one of those harnesses that clip to a baby seat mount at one end and you thread the existing seat belt through the other end. That works a treat, although when they get too old for the harness I'll have to think of something else. (Although come to think of it once they're too old for the harness they probably won't fit in the back seat anyway!) With the harness you really only need lap belts so that seemed ideal.
I fitted the baby seat mounts in the boot and threaded the harness through between the boot hinges and into the back seat. Only works with the top down though! I could take some photos of what I've done if there's interest.