Seems that me that changing the seatpost to alum. so you can safely attach some sort of collar to it would be the best way to go about this.
Axiom makes a rear rack called the Streamliner, which has a disc version, which may be an option, although you will still have to come up with a way to attach to the seatpost area.

Perhaps write to Spec. or ask at bike stores what rack they would recommend, perhaps there is a system that they have that would be simple, and not expensive. Given she will probably never be carrying more than 10kgs perhaps, this probably or hopefully gives more options.