Doesn't the 24V section of the furnace just control the main gas valve and ignition of the burner? At least on any of the furnaces I've ever worked with this is how they worked.
The blower is turned on by limit switches (120V) in the heat exchanger unit. Again this is how it is on the I've worked on.
The OP (Chris) is not modifying his existing furnace. He just wanted a pulse when the furnace turns on, and a different pulse when it turns off. The pulses will control a battery-powered RF remote transmitter which will turn on/off a ceiling-mounted paddle fan.