I promise that 757 packs do not overwork on ferry flights. I ferry empty 757s just as much as I fly full ones, and never has there been an overworked pack problem.
Two other possible causes of burning smells we have experienced in the 757:
1. Dust accumulation on top of the standby inverter begins to heat up and smolder, causing an unidentifiable source of smoke and a foul smell. Resulted in a diversion and a lot of pilot sweating over the Indian Ocean.
2. Electrical arcing on the screw connecting the window-heat power wires to the forward windows. Those screws are supposed to have a plastic cap on them, but sometimes the caps fall off and the screw is exposed. The arcing is enough to cause a very objectionable smell and two diversions so far, until the cause was finally discovered. Another note: that bare screw would really give you a jolt if you touched it with a metal clipboard by accident.