I suspect that it is dependent on the fixing in the seat frame, which in all probability is welded.
I converted a passenger seat in our old Rover to give height adjustment. This involved mucking about with the seat frame but fortunately it was just a case of a donor drivers seat and changing brackets. In all probability now days every thing will be welded. Maybe and I think it would be vetoed, a plate could be made with a lowered fixing welded to the plate and the existing bolt screwed into the existing frame mounting and the armrest bolted to the lowered fixing BUT that I would think would a) costly and b) a problem with the DOT type certificate.
I haven't used one but often in the past members have bought an armrest/cubby hole that sits between the seats.
Good hunting
