Robin,
I did spend some time looking into this problem, and it is a major problem in the Vitesse and Herald convertibles. If you mount the reel on the top of the diff (suitable strong mounting point in the boot) then in an accident there is no strength in the framework that the rear seat back is attached to, and the leverage and momentum of a person using the belt would flatten the seat frame which would then render the belts useless. The only way we could do it, is to put in a low level roll bar that follows the top of the seat ( to allow the hood bag and hood frame to fit as normal) and provides a strong mounting point for the reels. As there are safety implications the only way we could do this and sell them was to have a proper roll/safety cage company make them. The costs were coming out in excess of £250 not including fitting. Consequently the idea has been shelved.
There is one member in N.I. who has made his own version of this solution and I am afraid his name escapes me. I am sure the engineering was well done but for liability reasons we could not offer his plans through the shop. So you are unfortunately on your own unless enough people want the safety cages solution that the cost comes down.