I don't know about earlier models but the gearbox cover on the 2012 Plus 4 is an appalling bit of design.
The back of the gearbox cover has the transmission tunnel fitted over it. This means that the leather trim has to be peeled off and the transmission tunnel removed before the gearbox cover can be removed.
The front has a flange folded upwards that makes it impossible to get the cover off (the flange makes it impossible to slide the cover past the heater/radio box). If the flange was folded out from the firewall, in other words part of the firewall not the gearbox cover, the cover could be fitted over it and you could remove the cover without removing the heater/radio box. Apparently in some cars it can only just be forced past bottom of the heater box: not on mine
Given that the tail shaft can be accessed from under the car it is difficult to see any need to remove the transmission tunnel unless it has been damaged. In my opinion it should be possible to remove the gearbox cover without disturbing any other components. It is glued to everything with mastic anyway so water should not be a problem
better still would be some good neoprene strips to form gaskets.