I'm with Richard UK on this one, but for a different reason. I see no difficulty with the location, just the intricate design of the operating mechanism. As Richard says, it is easy enough to drop the diff or remove the calipers to work on them off the car. A lot of the bad reputation comes from people who have struggled to work on them in situ. I know that the mechanism shouldn't pose a problem if it has been properly looked after, but there are a lot of cars out there that haven't been, and then rebuilding the mechanism and finding replacement parts if required is not at all easy.
The P6 takes all the cornering loads up the drive shafts and through the diff into the bosyshell, rather than through the suspension as on a normal design. In fact braking and traction torque is resisted via the top and bottom links from the de dion elbow. But I do agree with Richard that it would be better to retain an inboard design.
I think the correct solution is to use different calipers with a more maintainable design. The key point being to separate the braking and handbrake functions. There are a number of proven solutions out there to achieve this. First up is Simon Owens solution of using a complete Jag diff, discs and calipers. Then Alan at Classeparts has done a number of conversions using Sierra calipers - standard to go round a non vented disc or Cosworth to go round a vented disc (which I prefer because of the inboard location). Problem with this set up (I don't know the numbers for the Jag, so can't comment) is that the braking effort Is increased relative to the Rover original. I define braking effort as piston area times pad area provided the discs are the same dia. In my view increasing rear braking effort is a bad idea - the brakes are plenty good enough overall already and greater effort at the rear only serves to destabilise the car under braking. It is possible to tackle this by varying pad material though - eg by using EBC red stuff pads or similar.
Alan also has a yet to be executed plan to use Audi calipers as well.
All these solutions finish up with a separate handbrake caliper acting on the same disc as the main caliper; which achieves the required simplification of mechanism.
Chris