I've added a relay that is energized when the thermostat calls for heat. This relay is switching the fan contacts at the thermostat so that the fan comes on the instant the thermostat calls for heat. I've done this as a sort of safety feature, so I'm not relying only on the furnace's internal bimetalic thermostat to turn on the fan.
Something else I was thinking of doing was putting one of these in series with the gas valve relay control:
That seems to be normally used only to turn on the humidifier motor when the furnace is actually running. Which is strange, since it would be simpler to power the humidifier transformer from the furnace fan power supply, which would insure the same result (that the humidifier motor runs only when the furnace fan does).
I don't see why that air pressure switch couldn't be placed in series with the gas valve wiring, so that the gas comes on only when the furnace fan is started and develops a positive pressure in the plenum.
So the way this all would work would be:
1) Wall thermostat calls for heat, closes relay that energizes gas valve. This also energizes new relay that starts fan.
2) Gas valve does not come on, because air pressure switch is currently in the open position (no plenum pressure).
3) After a few seconds of fan operation, pressure builds up in the plenum, closing the air pressure switch, which then allows the gas valve to open and flame-rollout happens, furnace generates heat, etc.
4) Wall thermostat at some point tells the furnace to turn off by de-energizing gas relay. This shuts down the flame and the relay that turns on the fan. But the fan will still be running, because the furnace thermostat will still be calling for the fan to operate until it reaches cool-down set point.