Full size aircraft use a RAT (Ram Air Turbine). This is a small windmill that sticks out into the airflow (not the jet pipe/exhaust gases). On full size aircraft this can provide electrical and hydraulic power. Personally, I would prefer this to having anything in the exhaust section.

I must admit that I don't actually know how much power you would lose by putting a propellor in the exhaust. Bear in mind that 2-3 cm is quite big in comparison with the size of the engine turbine section. (This obviously depends on which engine you are using).