A manual version of the Plus 6 would make a mockery of the extreme performance available with the auto trans. A five or six speed manual box could simply not be shifted fast enough even if you chose optimum revs to change. You have to appreciate how quick the 8 speed auto on this car is. I would be surprised and maybe even dissapointed if MMC provided the option.
Have a manual box on your Morgan by all means, but not this one.