An interesting one this, as I am sure it was an issue when it was first suggested females were to be trained as FJ crews.
I have asked the experts if they can provide an answer, and will report back if MB respond. I suspect there are 2 issues here:
1. Might the reduction in weight might actually increase the likeliehood of injury from the actual ejection? Light weight = greater acceleration forces = greater possibility of injury due to compression:
Force = 900N X Acceleration (15 stone person)
Force = 600N X Acceleration (10 stone person)
If force is constant, the rate of acceleration for the lighter person will be half as much again for the lighter person. Now, I believe the weight of the occupant is 'dialled-in' on modern ejection seats, presumably to compensate for this, but there must be limits as to how much the seat can compensate?
2. For specific areas of the flight envelope, the seat behaves in different ways, as explained by jimgriff on his website. However, for a particular situation, the seats must be expected to perform in a given manner. I suspect that the manufacturers 'guarantee' that the seat will perform correctly within certain limits, and outside those limits there are no guarantees. Perhaps they haven't been tested outside a given range of weights? Given 'Duty of Care', I doubt the RAF/RN will risk it.
Waits to be shot down by those who know far more about such things!