No problem in teaching an observer with average aptitude to do a run on landing (flat, straight approach) in about 10/12 hours. They could certainly fly/navigate to the nearest airfield with crash facilities. Night flying would probably require more training.
Problem comes with dual controls and re-current checking.
Dual controls could interfere with normal LHS duties and could be a hazard. The concept of dragging an incapacitated pilot away from the controls is a non-starter.
Checking would obviously cost time/money and there would be the inevitable CAA involvement - with all that entails.
Both the above are not insurmountable given commitment.
And - big bonus - it would certainly get rid of the current argument re observer status - passengers or crew....