Interesting thread Mark,
I've often wondered this myself.
For years here, houses have been using the traditional 2 pole main swith on the switch-board, one pole isolating the mains supply to the house, the other isolating the Hot-water supply.
However, we must not get confused with the service line fuse here, that is only designed to protect the service line itself, not what is on the end of it, the mere fact that it often does, makes people think that that is what it is installed for.
For years, a pole fuse was nothing more than a piece of 63A rated tinned copper wire, indeed, the NZI pole fuse holders were made for this.
Nowadays, where overhead reticulation is still used (and this is getting less and less every year), most pole fuses are slowly being upgraded to BS88 HRC fuses.
Now, when a place is underground reticulated, HRC fusing is used as standard:
Refer the pic below:
I really can't see why an MCB can't be used a s a Main Switch here.
In any well designed installation, the MCB (as the Main Switch) should never trip anyway.
Provided that the lower rated sub-circuits are co-ordinated with the I2t characteristics of the Main MCB, there should never be a trip.
From an electrical safety point of view, this idea is a great one.