Fixing disabled drop-down wc handrail to plasterboard wall

[Marlborough]

Problem arisen due to house construction. Going from interior to exterior construction is plasterboard,studwork,large gap,blockwork,timber frame,moisture barrier and finally harling. Obviously a drop-down handrail will be loadbearing and should be 700mm from floor when dropped down. I would be grateful for any advice as all convential methods have so far failed. Thanks. :?: