Yes, and yes. Soldering directly to pin 79 of the 160-pin chip always works, provided you do it correctly of course. Be VERY careful, it's very easy to snap the pin.
1) Lift pin 79 (i.e. use a soldering iron to heat the pad, then lever the pin up so it no longer makes contact with the board - a small pin or sewing needle can be used as a lever).
2) Solder a wire to pin 79 (tape the wire down so it doesn't move about) and solder the other end to the middle pin of a SPDT toggle switch.
3) Wire one of the outer pins of the toggle switch to the 0v pin of the power supply, and the other outer pin to the +5v pin of the power supply.