I dont believe your proposal would work unless there was the equivalent of a final exit available from the first floor lift lobby area, so that persons taking control of the lift and the evacuation were doing so from a safe area, accessible from the outside if further assistance were to be necessary.
You will note that the new guides are are a little more flexible on the use of lifts for evacuation of disabled people and move towards a risk based approach allowing decisions to be made as part of an emergency plan to use lifts elsewhere in the building if they cannot be affected by the fire. There are no recommendations made on what constitutes an adequate degree of separation, in terms of fire protection or electrical resilience.
I would not rule your proposal out entirely because the alternative - the building witout any evacuation lift, especially with a large number of non ambulant people using it will clearly be less safe than if we can use a lift to quickly get a larger number of persons to a lower level. To justify your case you will first have to apply the principles of prevention- can we find alternative accomodation for this department in another building on the ground floor?
If not, the second option is to rearrange the layout of the ground floor to make evacuation control operate from ground floor level, if that is impossible then look at lift control systems and management procedures. Can you provide a dual standard of lift control with the first stage of control taking charge at first floor level by key switch, but then moving to the ground floor with a second keyswitch that over rides the first?
