*I would certainly focus on Head/Neck Control; consider Cervical PNF Patterns Contract-Relax Patterns
*Have you tried Rhythmic Trunk Stabilization in Sitting to facilitate Trunk Control (i.e. Perturbations)
*Do you have available Ergometer equipment (i.e. UE/LE Cycle or Both) with available PROM options? Cardiovascular training is also important in this population.
*Orthoses considered for foot deformities?
*How advanced is the Wikipedia reference-linkScoliosis (<40 or >60); a brace may still be an option for the curvature as muscle weakness shows poor correlation with the idiopathic scoliosis observed in Friedrich's Ataxia.
*If your forced to PROM, I would certain attempt Bilateral PNF Flexion Patterns of the UE to facilitate Trunk Extension and Thoracic Expansion to address the issues of the Scoliosis and decrease the risk of cardiopulmonary dysfunctions secondary to the advanced curvature.

Keep me posted on your progress and I will see what I am able to contribute