We have operated small single engine aircraft in support of Parks Canada's Mountain Rescue program for 20+ years with no problems.
We use various lengths of fixed lines, attached to the aircraft with dual hooks. Vertical reference is used by the pilots to place rescuers on location. Slinging distance and time with live loads is kept to a minimum.
Altitudes in the Rockies reach 13,000', but typical rescues are preformed between 8,000 and 12,000'.
It would be great to have the twin engines and hoisting capability, but economics preclude this type of operation.