I really don't see how it can be the parking pawl - as that's on the motor input shaft and turns at 8x wheel speed. There also appear to be 10 indents per revolution for the pawl to lock into, so for every rotation of the wheel there are 80 opportunities for a misbehaving pawl to get snagged. (Or 8 if it only snagged at one point on the rotation of the input shaft) That is way, way faster than the repetition rate of the noise, by about an order of magnitude.
Also the noise is dependant on direction of torque not the direction of rotation - I can produce the noise in either rotation direction but only when the torque is in one direction. (driving in reverse under load or decelerating when driving forwards) As the parking pawl operates directly on the input shaft which the motor drives without passing through any gears first there is no way it can detect or be affected by direction of torque - it could only be affected by direction of rotation.
I've changed my mind on where the problem is likely to be - I don't think it's a bearing fault anymore, I think it will prove to be a chip out of the side of one of the teeth on the large differential wheel - that fits in with the noise happening once per wheel revolution and also fits in with the noise only occurring when the gears are meshing in one direction such that force is applied to the chipped surface. In the other direction the opposite undamaged side of the tooth is being driven so it remains silent.
If it is indeed a chipped tooth, reconditioning the gearbox is out of the question if Nissan don't make the gear wheel available as a separate part - while bearings are probably available after market the gear wheel most likely isn't and will be custom for the Leaf gearbox.
So box replacement is almost certainly the only option so I don't think there's any point trying to strip it down or even drain the oil at the moment to be honest. (Although if I retain the old box after another one is fitted I will probably open it to perform a post mortem to find out what the real cause was)
Incidentally I've had some further quotes come back since the initial £870 inc vat offer from Bury that I received by phone:
£850, 6 month guarantee - Southampton, no further details so I'd need to phone them to find out about the car it came out of
£3,654, 3 month guarantee - Rochdale, same as above no further details without phoning
£2,195, 24 month guarantee, "recon" box - Brentwood
If people are willing to charge up to £3600 for a 2nd hand box, I can only assume the new price is over £5k!!!
I think I better hurry up and get one of the ~£800 boxes while they're still available with some of the others being so expensive...

I'll give the one in Southhampton a call tomorrow to find out more details about the car it came out of to compare to the first one.
Once I have a box I can worry about who or how to install it.
One thing occurs to me - when pulling the driveshafts out and swapping the box, will the gearbox need new output shaft seals fitting ? Should I be trying to track down some new seals ? I assume a box from a scrapped car will still have the original seals in it.